As for decoration, you may have a theme based on their favourite cartoon/character.
Get some nice wall stickers and their favourite cartoon bedsheet.
My 8 years old son's bedroom have the following, for your reference:
- Queen bed + cupboard (the normal kind of set)
- a bedside small table (suppose for alarm clock, photo frame with his photo)
- kids writing table with chair (got this when he was 4yrs old.. kind of waste space and stuck there no longer use; he is doing his studies at our dining table)
- bookshelf (for story books collection)
- bookshelf (for school books and reference books)
- 4-level plastic drawers (for all his toys -- all dumb inside -- a mess)
- special space to display his toys (boys .. ugh!)
* not to forget a reachable place to hang their clothing (such as weared jeans that is not for laundry yet, etc)
Ok.. thanks for the last tip.. will most probably get a clothes hanger for them.