I'm with LIz, I do the same thing. Even at a young age they can still follow a simple direction when you ask them not to do something and I think this will be a valuable lesson learnt and even though I don't advocate that children should be hurt to reinforce a life lesson, sometimes you just can't stop something from happening kwim? So if it does happen you have to make the most of it and hope that it will serve as a reminder when you next have to tell them that what they are doing could hurt them.