Occasionally, for various reasons, I will address the children myself, politely explaining to them how horrible their behavior looks to a stranger.

Often, the sheer terror of a strange adult (which I am) speaking directly to them is enough to shut down the poor behavior instantly, and suddey their parent becomes the protector to them instead of the enemy.

If the parent is foolish enough to attempt to rebuke me, they can't without justifying the actions of Thier little monster, and that gives the little terror a green light for bad behaviour. At that point, I just dismiss myself and the parent to deal with the situation... after all, they created it, not their child.