Safety for the bathtub?

Has anyone ever heard of a product that will lock the bathtub faucet so the kids can't turn it on without me? My 4 year old wants so badly to be independent now and he gets himself ready in the morning and even tries to get his 2 yr. old brother ready.....brushes his teeth and everything. I am so worried that he will try to run a bath for them without waking me up (he's a very early riser). I turned the water heater down but I feel like it could still burn them. I'm looking for something I can put on the faucet that will lock it, or not let it turn past a certain point. Any suggestions would be greatly appreciated!