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.
Oversized single initial in kid's room. Could do this in any color and font. Wall opposite bookshelves is a bright orange accent wall that matches the paint inside. Or last name initial on family room or kitchen wall!
two twin beds. A curtain hung from that cute heading would disguise your "bedroom" if you chose. A curtain would work above as well for either a bed or built in shelves and hanging space for your clothing; I really love the idea of deep, built-in beds