Had I been the restaurant manager, I would have treated this as critically as an armed robbery. I would have photographed the situation, gone directly to the offending parent. Ordered her out of the restaurant in a loud voice, and threatened to have her arrested for tresspassing if there was any resistance at all, and made sure that the message contained that you cannot do this to my customers. I would then have followed her out of the restaurant, and apologized to the other patrons, offering something like a free desert or coffee. This is the sort of thing that can kill a restaurant quickly if it's allowed to happen without intervention.