After M.E.'s quiet time a few weeks ago I went into her room and found that she decorated the base of her window seat with blue crayon. It was a beautiful, creative masterpiece and I regretted having to tell her that coloring on walls is specifically prohibited and that we'd have to clean the crayon off the wall (before dad saw it).
My sister loves the Magic Eraser for such tasks but I've yet to buy my first Magic Eraser (yes, I'm that cheap. If you are shocked I'll send you my address and you can mail me one--ha ha).
Anyhow, if you are in this position and you don't have a Magic Eraser either, try to scrub the crayon off with EITHER (meaning, don't mix) ammonia on a clean, white cloth or a cloth and toothpaste. Which ever you try, make sure you rinse the wall off when you're finished.
Worked for me. Good luck getting it done!