how to build toy shelf near ceiling of kids room

Hi, i'm new here. I hope i'm posting correctly. I would like to put a shelf around the whole perimeter of my childs room. the shelf would be high on the wall, near the ceiling (except she has catheral ceiling so its not really near the ceiling but would be in a room with regular height ceilings) and basically just a flat shelf board with wooden, ready made supports underneth. I can figure out all the problems like making the miter cut in corner where boards meet and supporting the shelf board with the bracket boards underneth, but what eludes me is how the corners will be supported. I don'tthink there would be a bracket under there in the corner...i don't know. is it just that I'd have the brackets NEAR the corners and the boards just well supported for the weight that would meet in the corner? any ideas? thanks. oh, and also i'd put a molding just under the board shelf for a finished look and added support i'm sure.