I like the little day bed and the window and beat board walls. day bed on plywood base with built in table. we would put the long edge further back for under the bed storage space. I also would want to paint it like white or something.
Children's Bed - Looks Like A Little House Headboard . instructions seem to be in Dutch, you can try translating it into English but you get the idea for a cute bed that you could copy from the picture and the dimensions of a twin size bed.