Feel free to let the kids choose their own wall color, but remember to limit the selection to the ones you think can work and which are a bit timeless.
Kids rooms are often filled to the brim with toys and stuff. If you have too much clutter, it will make it difficult for the kid to choose and to start playing. Make sure you get rid of toys that they have grown from and is no longer in use and done to others. Create enough space for the toys that your kids need right now, that engage and stimulate.
Teach the children where things should be and make it easy for them to join in on the clean up. Kids are good at making a mess, but they can also be good at cleaning! It’s easy to get organized if everything has its own place.