Here is the method that we used with Weston, I read that it is supposed to imitate what the mother dog and littermates would do...When he put his open mouth on us (not even nipping necessarily) we would yelp loudly (like the puppy would if you accidently stepped on him), and then immediately get up and walk into another room, or into the corner of the room that you are in. Turn your back on him and completely ignore him for a couple of minutes. Then you can go back to playing. If he does this more than twice, we just quit playing completely.
It doesn't matter how fast the kids move, or how big they are. You/they need to establish that they are higher in the pecking order than he is.