Theory:

Whatif I’m dumb in school?
Whatif they’ve closed the swimming-pool?
Whatif I get beat up?
Whatif there’s poison in my cup?
Whatif I start to cry?
Whatif I get sick and die?
Whatif I flunk that test?
Whatif green hair grows on my chest?
Explanation:
 
The song begins with a series of 'whatifs'. Speaker worries about several situations that may or may not happen in his life. He thinks, "Whatif I’m dumb in school?". This question is important for various reasons. It gives us a hint as to who the speaker is: a kid. It also exposes one of the biggest worries of a kid, which is him turning into a stupid person or being treated as one at school.
 
cartoon-1300595_960_720.png
The speaker doesn't want to be dumb in school

Later, the speaker wonders about a sillier scenario: he imagines the swimming-pool to be closed and wonders what would he do in such cases. The speaker probably loves swimming and is worried that he may end up missing one of the sessions.
 
swimming-pool-149632_1280.png
Whatif the swimming-pool is closed?

The third whatif is about the situation where the speaker might get beaten up. It is a worry that many students have. The kind of beating mentioned in the poem might be a reference to the act of bullying or may mean punishment in general.
 
shutterstock_786294580.jpg
The speaker is worried that he will get beaten up
 
The poem also contains a series of other worries that are expected of a child. The speaker worries over situations such as failing a test or starting to cry. He also worries about the uncertain future when he might get sick and die.
 
sad-3176411_1280.png
The speaker worries about the time when he might get sick and die

The poem also contains a couple of sillier and crazier thoughts of a child. He wonders about a situation when he may end up finding poison in his cup, or worse, growing green hair on his chest.
 
oie_ZYBvCFNC4aWo.png
The speaker fears that he might grow green hair on his chest

The lines from "whatif I am dumb" to "my chest" contain a striking rhyme scheme AABB.
 
Meanings of difficult words:
 
Sl.No
Words
Meanings
1
DumbStupid; becoming mute; Temporarily unable to speak, for example, because you are very surprised or shocked
2
Beat upTo subject to repeated beatings or physical abuse
3
FlunkTo fail, especially in a course or an examination
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.