Hot glue rubbing plates. Make your design with hot glue on a square piece of cardboard, let dry and then hand it over to the kids with some sheets of paper and crayons to rub over.
If you need a super simple volcano recipe, it doesn't get much easier than this. Prep is about 3 minutes, and there is no clean up necessary. Easy Baking Soda and Vinegar Volcano Eruption for Kids by Preschool Inspirations