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.
Here's an innovative solution from Danish textile designer/potter Antero Kjaer: a couch/bed made with two twin mattresses, hooks, and foam. How To.Make a Couch, Bed, and Guest Room Fit In a Small Space