If you are talking about physical defense, put them in karate. My son is 5 and in karate so far he has learned how to escape from front choke, back choke, head lock and full nelson. He knows how to throw a proper punch and kick.
Now mental defense is something different. If your child is being teased or picked on, that can be harder to defend against. My approach is to tease and pick on my kids as much as possible at home, so they get used to letting things roll off their backs. Like when they ask for a hug I'll say something like "No, you're ugly and you smell funny." They brush it off, and of course I always give them the hug anyways. If your child hears nothing but praise all the time, they don't know how to react when someone says something mean. If they are used to ignoring rude or negative comments, than they are less likely to get upset if another kid says something mean to them.
at 4:14 PM on Jul. 4, 2009