Here are some of my favorite colorful wall decals. I think these are a brilliant design solution, especially in a child's bedroom as they are easy to switch as the child grows.
Tree with Birds from Couture Deco. I love their single-color decals. I think it would be really easy to plan a room around just one color.
Noah's African Circus from Pop & Lolli. On a neutral wall, these colorful animals would make anyone happy.
Big Blowing Cherry Blossom Tree from Surface Inspired. This one looks very elegant and would work for a girl at almost any age.
I've never used these but they are supposed to be easy to apply and easy to remove. Such a great way to spice up a room!