I immediately thought of OWL MOON, as well. But I would consider that free verse poetry, though.
Are you only looking for prose? Or did you just mean no rhyme?
Another one is BEACH FEET. Not sure if it's considered free verse, though it feels like it. I think it's a translation from Japanese.
ETA: Just wanted to add that the reason BEACH FEET touched me is that it is very much written from the child's perspective.