Theory:

Important!
The following material is provided for additional reading. The material provides an insight to the phrase "what if" from a linguistic perspective. Since the poem "Whatif" is structured around the conditional clause, a theoretical material has been provided to understand the same.
'What if' is a Conditional Clause. Conditional Clauses are also known as 'Conditional Sentences', 'If Clauses', and 'If Conditionals'. They are known as If Conditionals/clauses because most of the sentences using the conditional contain the word 'if'.
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What is a clause?
A clause is a group of words that includes a subject and a verb to form a simple sentence. It is  mostly used to identify a part of a whole sentence.
Example:
In the above definition of a clause, the line 'a clause is a group of words' is considered a clause, whereas the line 'a clause is a group of words that includes a subject and a verb to form a simple sentence' is considered a sentence.
What is 'a conditional sentence'?
A conditional sentence expresses a condition (a requirement or requisite) in a clause, phrase, conjunction, or verb form. That is, a conditional sentence consists of two parts- a main clause and a conditional clause. The conditional clause usually begins with words such as 'if' or 'unless'.
Example:
In the sentence "if you work hard, you will succeed", the clause "if you work hard" is an example of the conditional clause and  "you will succeed" is the main clause.
The conditional clause can come before or after the main clause.
 
For example, the above sentence can also be written as 'you will succeed if you study well'.
 
The 'If Conditionals' consider imagined or uncertain situations and the possible results of these situations. They are used to express that the action in the main clause (the part of the sentence without if) can only take place if a certain condition (in the clause with if) is fulfilled.
What If - conditionals
What if is used when the conditional sentence becomes a question. 'What if' is typically a question  which means 'what could happen'.
According to the Cambridge dictionary, what if is "used to ask about something that could happen in the future, especially something bad".
Being a part of an If conditional, 'What if conditional' also considers future with uncertainty, anxietywonder, or interest. However, being a question, the idea of 'what if' is to seek an answer for an imagined or an uncertain situation. Unlike the conditional clause, 'what if' questions do not contain two parts: it contains only the 'if clause'. Moreover, most of the what-if questions are rhetorical in nature. That is, a 'what if' question is asked in order to create a dramatic effect or to make a point rather than to get an answer.
Example:
An example of an If conditional containing two parts: If I skip breakfast, I will feel tired.
 
An example of a 'what-if conditional' containing a single part: "What if you skip breakfast?" Here, the expected action or outcome is not included.
Examples from the poem "Whatif"
Last night, while I lay thinking here,
Some Whatifs crawled inside my ear
And pranced and partied all night long
And sang their same old Whatif song:
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?
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?
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!
 
Important!
The lines in green are examples of 'what if conditionals'. On the other hand, the word 'whatif', in black and used thrice throughout the poem, is not an example of a conditional clause as it is used as a noun here.