I agree with 7to25
Go with something plain, maybe different colour paints and possibly a stripe wall paper. Children's rooms tend to be busy busy and full of stuff and they need to evolve. My son's room is currently on 3rd incarnation. He was 4 when we moved in (baby blue walls, chambray denim curtains, animal cushions). Then we added a deep red wall with some city at night framed drawings and a change of bedding to Stars and stripes type, now adding some black elements with a painted and wood desk to make it more grown-up. He's now 10.
£150 a roll!