I have seen trainers train their kids to say "Ouchie" when the dog nips them. If done properly and in a way that the sees it as a NEGATIVE and not a game (cus done wrong the kids will just come across as tho they're "sweeky toys" and that's starting a whole other bad thing...). You're kids need to be made aware of the sounds they make when they play with the dog. This way the negative sound that "Ouch" should sound like for the dog will be perfectly clear. It needs to be a distinctive negative sound that is only made when the dog nips or bites them. Let us know how it goes... Oh, and happy holidays!
