I think parents should be banned from kids' activities...Little League, dance class, riding lessons. Sheesh! What a rude woman. But try not to give her any room in your head. Best to continue on as if she doesn't exist. I think that is the best example you can set for the kids in your group.
However, if she does interfere or if her attitude does spill over to her kids then you may have to take steps for the benefit of the group. Hopefully that will not happen.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger