Equal force.
This happened several years ago when my 10 and 13 year olds were considerably youger, probably 6 and 9.
My black lab, Ted -- loveable goofball of a hunting dog -- was tied on the back porch, watching the kids play in the backyard. Our version of a security system, as his bark is loud and ferocious, despite his gentle personality. And yes, I was still checking on the kids regularly.
Well, I was just inside the door, and heard Ted bark like I've never heard him bark. I raced out the door to see an enormous mutt/shepherd/lab cross advancing across the yard. I don't know if he was just going to give my kids a sniff, or to eat them, but Ted intercepted the dog -- who stood almost twice as tall as him -- bowled him over by ramming him chest-to-chest, and held him down by the neck in the dominant-wolf position. I've never seen anything like it from my beloved, gentle hunting dog. I hollered for him, he released the mutt, which ran like hell out of the yard and across the road into the woods.
Though it appeared he wanted to kill the other dog for getting near "his kids", Ted didn't have any blood on his muzzle, and the kids were just standing there in shock. Even though he and my springer are both house dogs, I make sure when they're let outside on the rope that the rope is strong -- but not too strong.
Joel