a little off topic but you are all so creative

Hi, my daughter and grandchildren are moving in, and the 4 year old girl and the 5 year old boy will have to share a bedroom. I'd like to make the room special for them since they are having to get rid of so many of their things, but don't have a ton of money and I don't want it too childish so it grows with them. They will each have a twin bed, the walls are off white right now, and hard wood floor so any colors will be okay.

Any ideas would be greatly appreciated.