Theory:

Whatif the bus is late?
Whatif my teeth don’t grow in straight?
Whatif I tear my pants?
Whatif I never learn to dance?
Everything seems swell, and then
The night-time Whatif strikes again!
Explanation:
 
The speaker continue listing out his concerns. He is worried that his school bus would be late. Or that his teeth would not grow straight.
 
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Whatif the bus is late?
 
He also worried about the embarrassment that he would experience if his pants get torn.
 
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Whatif the speaker tears his pants?
 
The speaker is also worried about being unable to learn dance as dance is considered important for their prom night or parties.
 
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Dance is an important aspect of socialising
 
Later, when the whatif song gradually ends, the speaker seems to be glad. He says that everything seems great and happy during the day time, but when the night approaches and he is all set to sleep, the whatifs come and attack him again.
 
Meanings of difficult words:
 
Sl.No
Words
Meanings
1
SwellA piece of land that is covered or mostly covered with grass
2
StrikeA small, narrow river
Reference:
National Council of Educational Research and Training (2006). Honeysuckle. Whatif: 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.