This is a neat reading nook. I like the idea of curtains in place of doors, especially since this is a kids' room. Keep an eye out on them, but they feel like they still have privacy. This is a great idea for a playroom, too.
Great idea RE the twin mattresses. I also love the tree pictures on the wall! Love to display kid-made art in a kids room! Two twin mattresses, some plywood, and a great playroom that doubles as a guest room or sleepover room.