Awesome Loft Beds--Since you won't have a window on the one end of your upstairs, this could be a fun idea for the kids area. Not necessarily a bed, but a 'loft' on top of a 'hide-out' area---or something. The bookshelf stairs are a neat idea too.
This marine plywood desk is super easy to make and can be done in a day. Dress up a child's bedroom with a compact and practical desk. Mount recycled paint cans spray painted in bold Rust-Oleum spray paint colours for storage or display.