Actually, I used to teach elementary school & they would wash out but usually I had to use warm-hot water, soap & lots of scrubbing -- not good for quilts. The only thing worse were the Rose Art crayons (not labeled washable) that I've actually seen permanently stain clothing.
I use the Crayola Ultra Clean crayons. Usually I don't even need to wash them out. During the quilting process some will rub off, most of the rest I can either brush off or remove with a damp cloth. Or I can just toss the whole thing in the wash with some Tide and cold water on the gentle cycle and it comes out every single time.