I've never had anything like this occur in my presence, but your comment has given me an idea.
If I am ever subject to something of this nature, I'll look the mother square in the eye and tell her that if she doesn't take control of the situation immediately, I'll report her to Child Protective Services and let the State take responsibility for her vermin.
"I'll report her to Child Protective Services and let the State take responsibility for her vermin."
See, this is part of the problem of why people won't discipline their kids in public. Some nosy busybody calling CPS on a woman who swats her out of control 5 year old. Some over-zealous caseworker could make an innocent mother's life miserable.
CPS is inherently evil. The misery that CPS would inflict upon the woman with the out of control child would be completely out of proportion to the offense. It would be akin to capital punishment for speeding.