Theory:

     Now, children have at their command a secret language, foreign to grown-ups altogether, in which they fluently communicate with trees, flowers, animals, the sun and the moon, perhaps even with the Almighty. Using that special language, Saeeda addressed her remark to the last departing ray of the sun. “Dearest sister, do come tomorrow with lots of warmth and brightness. You see, my mother is ill and needs your help.”
 
     “Surely,” answered the light, “don’t look unhappy. We’ll be here at the fixed hour.”
Explanation:
 
Saeeda caught up with the last of the receding sunrays.
 
last ray.jpg
Saeeda found the last of the rays on her tree
 
Saeeda used a unique language to call out to the little ray. The author describes this language as something known only to the children. He calls it a secret language, and interestingly, they can talk to nature. The children can communicate with trees, flowers, animals, the sun, the moon, and even with God. Unfortunately, as they turn into adults, they forget the language. That is so because, as children, we tend to believe in the possibilities around us. However, as we grow older, our innocence and imagination are replaced with reasoning and intellect. It is not the latter is wrong, but rather that it essential that we strike a balance between both the sets of qualities.
 
Hence, when Saeeda spoke to the little ray in the secret language, the latter understood her. Addressing the ray as her dearest sister, Saeeda requested the ray to come the next day with a lot of warmth and brightness. She explained how her mother was very ill and needed the help of the rays to get better.
 
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Saeeda asked the little ray to come and help her mother to recover
 
The little ray answered back. She promised to be back tomorrow and asked Saeeda to be happy.
 
Important!
Note: As you could see, the rays are treated as a person here. This is an example of personification.
Meanings of difficult words:
  
No.
Word
Meanings
1
Command Proficiency or fluency in a language
2
Foreign Something which is less familiar or unknown
3
AlmightyGod
4
Fixed hourRegular or the usual time   
Reference:
National Council of Educational Research and Training (2006). A Pact With The Sun. (pp. 28-32). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.