Rhyme, of course, is the primary culprit. It adds a sing-song quality (sometimes unintended) to work. Most poetry for children does rhyme so it doesn’t help perceptions.
But I would argue the vast majority of rap songs are rhyming poetry and you wouldn’t sit your child down to listen to Eminem. There are scholars who study Shakespeare’s work and I couldn’t accuse them of childish study.
Recently, it also came to my attention that people are denouncing concrete poetry as strictly “for kids”. Now, I understand the inclination to do this but don’t. Just don’t. The image part of the poem is an enhancement to draw you in before you even begin to read. Not everything with pictures is for children.
I wish I could say the folks putting these forms of poetry into one box were not educators/editors/publishers/poets but that’s not the case. Even industry veterans are decrying the “childish forms”. I thought the age group for a poem was dependent upon the subject matter and how it is presented and, even then, it’s not always set.
Never let someone tell you what to write or how to write it.