I'd start by asking yourself how much you want to get paid for your time? $10/hour, $50/hour, $100/hour? Tally all of the hours that will go into this project from start to finish (making cookies, gathering supplies for all, traveling, setting up the space for the kids, cleanup, etc), multiply by your hourly labor rate, add the cost of materials and gas, and then mark up so you make some margin on all of the above. It's really the same process for pricing out anything.
Price should be driven by the number of hours you invest and less so by the number of students, as there is a large amount of fixed prep time that does not vary much with number of students when setting up for a class.
A more cost effective solution for the client would be bringing the kids to you, as few people are going to want to pay for 3 hours of driving time. But I would charge for it.