I've used washable crayola crayons and markers, both of which I use very lightly so at not to sink the color very deep in the fabric (I do have things that were "crayola washable" that stained because of my daughter's intense coloring, mostly I think on the rags I used to clean the intense coloring). I don't have any trouble washing it out when I do it lightly as I mentioned above, also I would say 90%+ of the things I'm drawing for end up in the seam allowance anyway.
Maybe write a little on a sample and then wash it? Oh and don't forget crayola sells both washable and stuff not labeled as washable! When you do a test sample don't forget no pretreating, because you wouldn't be pretreating an entire quilt...