I'd do a mix of money and special privileges, probably more privileges to keep the focus off acquiring money. After the kiddos are done their regular chores they can pick a work for hire job to get money.
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