I'm coming in late on this. I use regular crayons, but I have found that Crayola is the best. Others can be more waxy and will wash out.
I do put towels under but I put wax paper on top over the crayon coloring, put a pressing cloth over that and heat set with an iron. This was what I learned in a class.
This kind of heat setting of crayons will set it in fabrics, and wood. the fabric crayons will not heat set the same.
Give this a try. It will work. I promise. :)