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.
For a modern take on a traditional canopy bed, try bed curtains. Instead of being mounted on a canopy bed, these curtains look great hung from the ceiling. You can use a combination of standard curtain rods mounted on the ceiling to create this look.