What do you guys think about a little science type hut where kids can cast and paint their own fossils from pre-selected molds? This would need to be supervised of course, and to cover the costs of the plaster and paint used, maybe a small fee of £1 - £1.50 could be charged. Then the kids can take home their hand-made little treasures.
The molds would be rubber, but the plaster powder can be quite dusty if not used carefully, so this is where a parent or staff member would be nescessary.
I don't think you need to charge for the materials as there's already the entry fee, and how much does plaster really cost anyway? Might also be too dusty for kids, even if they are supervised by parents. Maybe they could be pre-made and kids only have to paint them?