What an awesome idea! This would be great built in a kids walk-in closet! Just take the doors off the closet and utilize the extra space! Keep the kids clothes etc in a wardrobe in front of the closet to make it a secret hideout!
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.