Are you talking about rice cereal or plain cooking rice? Maybe use brown rice with the husk still on it. You have to soak that a long while before it becomes lunch, I think. Or... yeah, use rice cereal or something edible. Actually, rice cereal sounds like a really good option, especially if your youngest begins walking and getting into everything and wanting to play with his siblings while still putting everything into his mouth.
Unless of course they're getting dirty and mixing real sand in there anyways. You don't want to invite them to eat dirty rice cereal.
I would honestly say though, keep your separate play area for it, have them go to it with clean hands, and give them something edible they can mix themselves.
...Now I want one for my son.