protecting children from nipping

Question
What specific immediate action should our 8 year old son take to protect himself when the dog starts nipping at his legs? The dog is a lab / Aussie shepherd mix, 6 months old and 35 lbs. We got the dog from a local shelter where he was returned by his original owner. He was in the shelter for only a week. We have had the dog for 3 weeks. He is a good puppy, with a good temperament but under trained. We have already started a family companion training class with the whole family (Dad, Mom, 2 boys,age 11 & 8) and using positive reinforcement training. He is responding well. The issue is that the dog gets excited when playing outside and will at a moments notice go from acceptable activities like fetching a ball to nipping at the back of your legs. Or will be running along side you and then decide he is going to nip. The problem is with our 8 year old. The rest of us can protect ourselves from him using the methods of negative punishment like ignoring him or stopping the activity. But this does not protect an 8 year old. the dog has ripped through his snow pants and broken his skin on more than one occasion as I race across the yard to get the dog away from him. No one tells us what to do on this occasion. The dog and children are supervised, we are using positive training methods but no one says what to do immediately so the child can protect himself? Will using positive punishment undermine our other training efforts? I am thinking of gving the 8 year old his rubber coated plastic wiffle ball bat to protect himself with a whack to the behind. I am looking for a solution as the 8 year old thinks the family dog is Jekyll and Hyde and getting scared of the dog.

Answer
It sounds like what you have is the herding dog's hard wired need to control motion (am assuming your son is often running when this happens?) and the retriever's hard wired need to have his mouth on something, or something in his mouth.  The classes sound great, and that, along with maturity should go a long way. Once the pup knows the name for behaviors, you can ask for a down or a leave it, or something else which is not compatible with biting pant legs. Because this behavior is self-reinforcing, and getting to do it creates a habit, I'd probably have the dog on leash when your son is running and playing. Any "grabbiness" might get him an immediate time out. I would also keep games with the puppy on the quiet and thoughtful side. Do some shaping with him, teach some tricks. Running and chasing, etc. is very stimulating, and it's very likely that when the pup gets wound up, that's when he starts redirecting towards pantlegs. So, keep him in the thinking part of his brain, and manage him when 8 year olds are playing like small boys will play. When you see him starting to go over threshold - getting grabby, snatching treats hard, that's time lower the energy of the games, give him some more thoughtful activity to do. Other possibilities are having him wear a muzzle during highly stimulating play or teaching him to always carry a kong on a rope or a wubba during these times.
As to the whiffle bat, I'm hoping that was a joke. Hitting the dog with even something which isn't all that hard sets up defensiveness, could turn your good-natured but hard-mouthed puppy anxious or fearful of your son. Then you have a problem that is a whole lot worse. An eight-year-old, especially one who is already a bit afraid of the pup, is not going to always be able to use good judgement in defending himself, and I'm not sure it's a great idea to teach a child that hitting animals is a good way to train them. Sandy Case BFA MEd CPDT www.positivelycanine.com