http://www.petsmart.com/product/index.jsp?productId=2754854The Halti could help in two ways: one, when the dog lunges, the Halti closes around his mouth, which offers some protection to the little dogs; two, when the dog pulls on the leash, the Halti pulls from below his chin and it causes him to go backwards or sit down.
I have five dogs, three of them as big as yours or bigger. My husband and I took all five for a walk once when a small dog charged us. The biggest ones are not as well trained as yours and they really wanted to go after that dog but because of the Haltis, we were able to restrain them and move on down the road.
I'm also wondering if you can carry some pepper spray or whatever is legal -- I know they make a spray for cyclists to carry with them. Maybe ask the Humane Society if you are allowed to spray a dog that is charging you and your dogs.
As a last resort, you could walk your dogs with muzzles on them, though that doesn't seem fair to your dogs and makes it look like they are the aggressors.
My suggestions are focused on those things that you can control because it sounds like you've done your best to influence the behavior of the dog owners and that hasn't worked. You are to be commended for caring more about the little dogs than what your liability may be. Good luck! :hi: