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!
I've just found Spot High Sleeper Storage Kids Bed. This unique kids bunkbed comes with built-in shelves and drawers, and a walk-in closet space, illuminated with LED lights - perfect for mini-fashionistas, or den-builders!