Hi, I have done a couple of quilts using crayons, one with my children and my niece and nephew for my mother's 65th birthday, and one I made with my DD's grade 7 class for an auction.
I cannot remember if I used a synthetic, a synthetic blend, or just regular cotton. But I do know I experimented first, so that is what I would suggest. Play with the crayons on a piece of fabric that you want to use them on.