Theory:

It was a beautiful paradise-like garden where children used to play every afternoon once they return from school. Children loved that garden as there was no other better place for them to play near their location. But, unfortunately, it belonged to a Giant who was so selfish.
 
The garden looked so attractive for its extraordinary ambience. It was a beautiful, big garden filled with soft green grass and trees, and the pretty lawn grass had some beautiful star-like flowers on it. Twelve peach trees gave out delicate pink and pearl flowers during the spring and produced rich fruits during autumn. Cute little birds had occupied those wonderful peach trees and sang sweet songs which were pleasing to the children's ears. Those birds' songs were so melodious that they sometimes made children stop playing and listening to them. The children had happily spent their time inside the garden.
 
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Children in the giant's garden
 
But, unfortunately, their happiness didn't last for a longer period. The so called-owner of the garden came back. As mentioned earlier, the garden owner was a selfish giant who lacked humanity and kindness.
 
For all those seven years, he stayed along with his friend Cornish ogre. 'Cornish ogre' is a Giant of a country named Cornwall in the U.K.
 
On returning from his friend's place, he was shocked to see children playing inside his garden as he was a selfish person. He couldn't tolerate them playing there. So he did not want to allow anyone to enter his garden. On being angry, he screamed at them, asking, "What are you doing here?".

The giant's voice scared all the children playing in the garden. So they ran away, thinking not to invite trouble for themselves.
 
The giant insisted that it was his garden. He claimed to permit no one to play in his garden as it belonged to him. He wanted to have the garden only for himself, and so, he had chased children out of the garden. This harsh attitude reflects his lack of gentleness, courtesy and humanity. Then he constructed a high wall around his garden to avoid people entering it, especially children. He believed that the wall would prevent people from entering his garden without his notice. He also put up a noticeboard: TRESPASSERS WILL BE PROSECUTED.
 
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The Selfish giant
 
The giant was merciless and selfish as he did not even let children play in his garden. The giant's ruthless activity affected kids. Children were upset as they had nowhere to play.

The children were no longer able to play, and they had nowhere to go. They tried playing on the road, but it was muddy and full of sharp stones, which they disliked. They used to talk about the lovely garden, and since they had no other option to play, they wandered around the garden's high wall after their classes were over. All they did was to recall the days they had spent playing inside the giant's garden and exclaim, "How glad we were there!".
 
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Children's recall on happy days spent inside the garden
 
Though they wanted to play there, they couldn't do so as, firstly, they were terrified of the giant; secondly, the huge wall around the garden had prevented them from entering the garden even during the giant's absence.
 
Days passed. Nothing brought a change in the giant's character. Then came a spring season when a strange thing happened. While the whole country enjoyed the perfect spring temperature, winter filled the giant's garden. Nature punished him for his arrogance and selfishness. Since there were no children around, the birds didn't like to sing, and the trees forgot to blossom.

A lovely flower had once emerged from the grass, but it felt so sad for the children's absence. It went to sleep upon seeing the noticeboard of the giant. The author here added this line to bring out the reason for readers why the giant's garden alone had suffered from winter. The Snow and the Frost were the only happy ones as they enjoyed prevailing the whole garden for a longer period. "Spring has forgotten this garden," they exclaimed and decided to occupy the giant's garden the whole year. The spring season is considered the perfect season of the year since it is the season of new beginnings.
 
During the spring season, flowers would bloom everywhere. Earth will have a new life again during spring. Also, the spring season has more sunny hours compared with other seasons. Here, we can realise the author compares children to flowers without whom the garden remains unhappy, affected by the winter season. When spring decided not to enter the giant's garden, it was winter that completely took over the giant's garden, which was nature's punishment. Grasses and trees left frozen with Snow. The whole garden was filled with Snow.
 
Snow and Frost then invited North Wind to stay with them in the giant's garden. The term North Wind in here clues on never-ending cold weather. It blew the chimney-pots down. Then, they invited Hail to the garden. Hail refers to small balls of ice that comes from the sky. The Hail knocked on the castle roof for three hours a day and broke most of the slates, and then he dashed as hard as he could across the garden. His breath was like ice, and he was dressed in grey. In this lesson, Oscar Wilde had personified nature, flowers, seasons etc., which were portrayed to be punishing the selfish giant for his selfish deed.
 
The selfish giant did not understand the strangeness of nature. He wondered why the spring had missed occupying his garden alone. He sat near his window, looking at his winter garden and hoped for the arrival of spring.

His expectation never came true. Nature did not show any mercy upon the merciless giant who failed to treat the kids gently. Still, the giant did not realise his mistake that it was his selfishness that resulted in the endless winter season in his garden. Not just the spring but the autumn and summer had not entered his garden.

During autumn, the entire country was enriched with fruits. But, the giant's garden had none of them as the plants and trees on his garden didn't blossom. So, neither the spring nor autumn entered the giant's garden, for they wanted to punish his selfishness.

His garden became the place for the North Wind, Hail, Frost, and Snow, and they danced about through the trees.
 
As mentioned earlier, winter took over the giant's garden completely. The selfish giant didn't understand the reason behind the winter's endurance in his garden. It was his selfishness that resulted in such a terrible result. Days passed. One day, he woke up from his sleep listening to some sweet, lovely music that he never heard before. He remained in his bed and was listening to that melodious music, thinking it was made by the king’s musicians who passed by that road.
 
Actually, it was the music made by a linnet bird. It sang in the giant's garden happily for some time, which the giant thought to be the music of the king's musicians. Since spring had not entered his garden for many days, he forgot the voice of the birds. For him, the song of the bird seemed to be the sweetest music in the world. The endurance of winter in the giant's garden has come to an end.
 
Everything, including the North Wind, Hail, Snow and Frost, came to an end. The flowers began to blossom, and a pleasant perfume reached him through the window. The giant guessed that the winter had come to an end. He then happily jumped from his bed to observe the garden from his window. He was happy to see the winter's end in his garden after many months. He exclaimed in a delightful voice as "I believe the Spring has come at last".
 
There was a reason behind the spring's arrival in the giant's garden: children had come back to play in the giant's garden. They secretly entered his garden through a small hole in the wall. When the children could not notice the giant's presence in the garden, they started to play inside it. When they came back, flowers started to bloom happily, spreading their fragrance throughout the giant's garden.
 
In each tree, a child was sitting and playing on the branch of it. The trees blossomed as they were happy to have children back in the garden.

As the trees started blooming, birds entered the giant's garden and were chirping around happily. The grasses have also begun to flower. The garden looked attractive and lovely. Still, the farthest corner stood a tree filled with snow and frost. Under the tree, a little boy was trying to reach the tree's branch.
 
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The little boy
 
Since he was too small, he couldn't climb up the tree and so started crying. The tree bent down as much as it could to help the boy climb on it, but it was of no use as the boy was tiny.
 
The giant was pleased to look at that beautiful sight on the spring morning. He was surprised with his garden's ambience filled with a pleasant fragrance, sweet music of the linnets and delightful kids. Only after having seen this sight, he realised his mistake for being selfish all these days. "How selfish I have been!" he thought to himself. He then understood the reason as to why the spring had not entered his garden despite occupying the whole country.

Having regretted his mistake, he wanted to do something. Also, he desired to capture the spring in his garden.

He decided to help the little boy who was crying under the frozen tree. He thought of putting him onto the tree branch to play. In this way, he felt, spring would come back to that tree. Moreover, he was much determined to break the walls and to let children enjoy the joy of the garden forever.
 
The giant silently entered the garden from his castle, opened the front door, and stepped out into the garden. However, children noticed him. They were scared to see the giant as they already had a bad experience on being chased and shouted at by him. They were reminded of that incident and ran away from that place. Surprisingly, spring went behind them, and winter occupied the giant's garden again.
 
To his surprise, the little boy who was trying to climb up the tree did not run along with them. The little boy's eyes were filled with tears that prevented him from seeing the giant. The giant had mercy on the little boy and went behind him, secretly thinking to help the boy. He went behind the little boy secretly and dropped him on the tree. Suddenly, a miracle happened. The moment he had dropped the little boy on the tree, it started to blossom; beautiful little birds came back to the garden, chirping around. The little boy was overwhelmed with the giant's help. Sitting on the branch of the tree, he extended his cute little hands toward the giant, flinging them around his neck. He was not as scared of the giant as other children. He kissed the giant to show his love as the giant helped him sit on the tree branch.
 
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The lovable giant
 
The other children who made their way to escape from the garden looked back and were stunned to see the incident.
 
They were delighted to see the giant allowing the little kid to sit and play on the tree. And when the children understood that the giant was not wicked anymore, they returned to the garden.
 
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The children's return
 
Again, the miracle happened. With the children came back the spring to the giant's garden once again.
 
The giant happily spoke to the kids. Then, he announced to them, "It is your garden now, little children". Earlier, he shouted at the kids saying, "My garden is my own garden". Now he understood his mistake and took a step to restore his mistake. Also, he took a great axe and broke the garden wall which he constructed earlier.
 
Around noon, when people were going to market, they were exceedingly amazed by the sight of the giant playing with kids in his garden. It seemed to be a pleasant sight in that most beautiful garden.
 
After months of waiting, the children were now able to play in the giant's garden, where they happily played all day until evening. Finally, they said their goodbyes to the giant as they were no longer afraid of him. The giant was happy, but his eyes started looking for the little boy he had put on the tree. The boy couldn't be seen anywhere in the garden or among the children's group. Since the little boy was missing from the children group, he inquired for him.
 
He was the one the giant loved the most among the children. After all, he was the one who kissed the giant and brought love in his ruthless heart. The little boy made him understand the importance of selfless love.

Surprisingly, no one knew about the little kid. He requested them to inform the little boy to come back to the garden the next day, to which they answered that he was new to the garden and that nobody knew about him. They remained clueless as to where that little boy lived as they had never seen him before.

The giant was highly disappointed to hear that answer. Nevertheless, his heart wished to see him again.
 
Since the children remained fearless of the giant, they went to the garden every day after their school got over and played there happily. The giant's heart had longed for the little boy's presence in his garden. Yet, the little boy never came to the garden again. Still, the giant treated the other kids with love and care and found happiness amid their presence. He often spoke about the little boy to all the children and explained his desire to see the little boy. But, unfortunately, his wish was never fulfilled.

Many years were gone. The giant became older and weaker. He could no longer play with the kids as he was old. Instead, he observed the kids playing in the garden by sitting on a huge armchair and admired his beautiful garden. Then he said to himself that the children were the most beautiful flowers of all, though he had many flowers in his garden.
 
Though the giant's eagerness toward meeting the little boy didn't succeed, he couldn't forget him in his life. Still, he always thought about the little boy he met many years before. Then, at last, one day, his wish came true. It was a winter morning, and he looked out from his window while dressing.

As it was a winter day, he didn't go out but was watched the winter's effect from his window. The giant was matured enough when he becomes old as he no longer hated winter. He didn't dislike the winter anymore since he knew it was just a matter of time before spring awakens and that the flowers were resting without blooming.

Suddenly, he saw something which he couldn't believe. Since he didn't believe his eyes, he rubbed them again and again. It was an incredible sight. A tree in the far corner of the garden was almost covered in lovely white blossoms. It had golden branches with silver fruit hanging from them, and underneath it stood the little boy he had loved and longed for. It was a heavenly sight.

His long term wish became true. He was filled with joy and excitement. He wanted to take the little boy in his hands; he wanted to spend time with him; enjoy playing with him. Finally, he could no longer wait and hurried to go near the beloved kid of him.

He rushed through the grass to go near the boy but was angry when he reached him. Two nail prints were found on the palms of the child and two on the little feet. The giant was angry about seeing those nail prints. He angrily inquired the little boy as to who had made those wounds on the kid's body.
 
The giant couldn't tolerate the little kid being hurt. After all, he was the favourite of all children he had seen in all those years. He was the beloved kid who stole the giant's heart forever. He was the one who brought love and mercy upon the giant's heart. So, he no longer waited but shouted angrily, enquiring the little boy as to who made those wounds on his little palms and feet. Then, he said he would kill that person with his sword who have hurt the little kid.

The cute little kid replied that it was not a mark of injury but a wound that indicated love. When the giant asked the little boy as who he was? A strange awe fell on him.
 
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Christ as the little boy
  
It was Jesus Christ himself who appeared in his garden as a little boy before years.
 
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Christ inside the giant's garden
 
When the little child revealed his identity to him, the giant knelt before him. The nail prints on the hands and feet remind us of Jesus Christ, who got crucified for the sake of saving us from worldly sins. Similarly, when the giant was selfish, it was the little boy who brought love inside his heart and made him realise the importance of selfless love, thereby saving him from the sin of selfishness.
 
The little child smiled at the giant. He told the giant that since he allowed him to play in the garden once, now he came to take him to his place, which was paradise.
 
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God among children
 
God gave him space in heaven as a gift of his selfless love he showed on the children all the years.
 
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God inviting giant to the paradise
 
The giant was found to be dead when the children entered his garden that afternoon. His body was covered with white blossoms under the tree. White flowers symbolise 'Purity'. As God was happy about his selfless life in the world, he gave space for his pure soul to rest in heaven.