Theory:

     All day long they played, and in the evening they came to the Giant to bid him good-bye.
 
     “But where is your little companion?” he said; “the boy I put into the tree?” The Giant loved him the best because he had kissed him.
 
     “We don’t know,” answered the children. “He has gone away.”
 
     “You must tell him to be sure and come tomorrow,” said the Giant. But the children said that they did not know where he lived, and had never seen him before; and the Giant felt very sad.
Explanation:
 
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.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
DisappointTo make somebody feel sad
2.
CompanionA friend or a partner
3.
Clueless Not able to understand
Reference:
National Council of Educational Research and Training (2008). The Selfish Giant – Oscar Wilde (pp. 17-24). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.