If the mother has acknowledged that its an issue previously and still hasn't done anything about it, then I think you need to be a little sterner in dealing with her and her child. I don't interfere initially when kids in the playground don't share properly with my girls, push in on the slide etc but do reinforce to the girls that that child is doing the wrong thing. If it continues I will say to the child that they need to share/wait their turn and then it steps up to saying it loudly that hopefully their mother gets the hint and intervenes. However, if one of my kids is being physically hurt the kid gets one chance and that's absolutely it. I would be telling the mother that she needs to do something about her kid's behaviour, and I would definitely be shielding my child from hers like Epacris suggested. Kids need to learn to fight their own battles, but they are also entitled to expect their mum to get in there for them when it goes too far. Its a difficult situation given you are in a small town, but social niceties and whatever difficulties the other family is having need to take second place to your DD's welfare, and if other mums have stopped play dates for the same reason you'll probably find you get a lot of support.