Do you have a little budding Picasso, and your walls are his canvas? No need to repaint or scream. (However a time out might be in order.) The tool for this project is right in your bathroom -and you can get if for free with coupons. TOOTHPASTE.
Dampen a clean, white cloth, put a little toothpaste on it, and scrub. I find that using my index finger works the best. The abrasives in the toothpaste scrub the crayon off the wall and leave it looking good as new.