I'm an old Girl Scout leader. I've used just about anything you can thing of for little girls to mark on fabric with. Crayolas are washable after heat setting. I have done many sweatshirts with them over the years. There will be minor fading the first time you wash and then I can say they will still look pretty good after being worn and washed regularly for years.
The crayons for fabric work well also. We've used those in many projects also.