Hair-dryer and Dish Soap
This combination can work wonders for you. As you may or may not know, most crayons sold today are made from coloured wax. When you apply enough heat to wax, it begins to melt, making it much easier to clean. Use your hair-dryer to heat the crayon on the walls until it begins to liquefy and melt. Place some mild dish soap (no harsh detergents or bleach) on a clean cloth or solid paper towel and gently wipe the wet crayon away.
WD-40 is revered for its many uses as a lubricant; however it is also a very effective method of removing crayons from walls. Simply spray WD-40 onto a clean cloth and rub off the crayon. Be forewarned, the older the crayon the more effort it may take. Once the crayon has been removed, wet a towel with water and go over the area to ensure no residue from the WD-40 remains.
White vinegar if used properly can literally be the MVP of your household cleaning arsenal. One of its many uses is to get crayon off of walls. Simply dip an old toothbrush in vinegar and get to work on those crayon marks. The scrubbing along with the acid in the vinegar will break down the stain. Wipe the area clean with a towel dampened with water.