From this, I take it that he knows, at least, that a poem has a different form and is typically shorter than stories.
Poetry can indeed be difficult to explain to children.
Rhymed verse is a cinch, but, of course, poetry is more than that and doesn't always rhyme. What's more, poetry for children is a specialty in itself. A couple of hundred years ago, it was tedious stuff whose goal was to instill religious and moral values. Then, thankfully, along came the likes of Robert Louis Stevenson, Lewis Carroll, and Edward Lear, who added fun along with puns. Nowadays, some of the best children's poetry is full of whimsical spontaneity, exhibited by masters such as Jack Prelutsky and Shel Silverstein.