You could also tweak this idea and use plastic plates as a base to make ponds, cakes, pizzas, the view through a telescope or portal window of an alien space craft. Think about what sort of moulds you have in your kit, what color clay you have, what things your children like, and then look for plates to suit.
When it comes time to make the craft, don't just walk away (or at least not straight away), take a moment to ask them what kind of things they might expect to find in a fish tank / outer space / pizza pie, and give them some hints on how to fashion up said items. Remember that the more successful they are at making things, the more satisfaction everyone is going to get out of the crafting journey. Praise the good things, try not to over correct too much of the weird blobby things, although I'm a believer in gently helping them to improve things within reason from time to time (control freak that I may just possibly be), and even take a photo of some of the items they're most proud of before you pack it all away.
Most of all... you know it; have fun!
Linda x