OK, I have used 100% cotton that was pre-washed.
The crayons are Crayola and are NOT specifically marked as washable. I sampled 23 of the box of 24.
I heat set it on the cotton setting. I placed the fabric so the crayon markings were face down on white copier paper and heat set it from the back. It did leave some residue on the white paper. I thought about using news paper, but was afraid the higher heat might cause ink transfer. I would suggest plain paper for the heat setting.
It will be washed with powdered detergent on warm, no fabric softener, and dried on medium.

In about an hour and 15 minutes I'll have the final results of our crayon test.

Mad Scientist Cathy