One of the quickest ways to update an entire room is to use some cool wallpaper - or made-to-measure murals. These wall murals are so easy to use, and all you have to do is find the exact size you need, get some wallpaper glue, and off you go!
I have found some cool designs that would look great on the wall in the girl's room. These are for 'bigger' tween girls, even teens or perhaps in your own space. Mr. Perswall offers amazing murals in many different styles. I have found a few that would simply be perfect for a tween room.