Call Back Yesterday by Nancy Kress -- juveniles in a mental health facility without windows suspect things are not as they look.
I would have liked Ian R. MacLeod's The Hob Carpet except, dear reader, for the florid and protracted descriptions in a prolonged narrative missive which invoked a manifest termination. (Okay, I frequently know the ends of stories near the beginning, but I think a six-year-old would get this. It would have been better as a pointed short story.)
A new author, Derek Kunsken, made a good beginning with Beneath Sunlit Shallows. I'll be looking for his next.