Theory:

Whatif nobody likes me?
Whatif a bolt of lightning strikes me?
Whatif I don’t grow taller?
Whatif my head starts getting smaller?
Whatif the fish won’t bite?
Whatif the wind tears up my kite?
Whatif they start a war?
Whatif my parents get divorced?
Explanation:
 
The whatifs continue to appear in the mind of the speaker. He wonders about a series of unfortunate events that may not even happen to him. For instance, one could see that he is worried over the possibility of his head getting smaller. This is indeed an absurd thing to think.
 
shutterstock_159163268.jpg
The speaker is worried that his head might grow smaller
 
The speaker is also worried over possible situations such as his parents getting divorced, unable to grow any taller, his kite being destroyed by wind, or even him not being liked by anyone. Such thoughts are common, and even natural, to kids.
 
shutterstock_533825065.jpg
Some of the worries such as kite getting destroyed by wind are common to the kids
 
Some of the whatifs are also pointless in nature as the speaker can be seen worrying over situations such as a flash of lightning hitting him, the fish not biting, and even the beginning of a war. Though pointless and silly, these worries exposes the imaginative world of the speaker.
 
pexels-philippe-donn-1114690.jpg
The speaker fears that he might get hit by a flash of lightning
 
It is interesting to see how the whatifs are a blend of serious and silly questions and concerns.
 
Another peculiar feature of the explained part of the poem is that the rhyme scheme is not as same as the rest of the poem. The \(1\)st two lines contain the same word at the end, the next two lines have a similar form of 'er' words in the end. While the \(3\)rd couplet contains rhyming lines, and the final two lines have no rhyme at all.
 
Meanings of difficult words:
 
Sl.No
Words
Meanings
1
BoltA flash (of lightning); a sudden or unexpected event
2
WarArmed fighting between two or more countries or groups
3
DivorcedNo longer married because the marriage has been legally dissolved; separated
Reference:
National Council of Educational Research and Training (2006). Honeysuckle. Whatifs: Shel Silverstein (pp- 121 - 122). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.