What was horrid poem in Henry has nits?

It seems you're asking about the poem in the book "Henry Has Nits" by Colin McNaughton. There isn't actually a poem in that book.

It's a humorous children's book about a boy named Henry who gets head lice. The book focuses on the humorous and slightly gross aspects of this situation.

Perhaps you are thinking of a different book? Could you provide more context or details about the poem you're looking for?