Crossing lines

In a recent copy of this magazine, there was an interesting article about whether or not it was appropriate to discipline OTHER people's children. If you see a child misbehaving, is it appropriate to step in and correct the behavior? Or, is it grossly insulting to the other parent--is it off-limits, taboo? I have to admit I've had my own feathers ruffled when well-meaning people have stepped in to correct my own children, or to comment on their behavior. And I do think there's a line between making judgmental comments about situations you don't understand as an outsider(this happens to us a lot), and stepping in to correct a child's misbehavior because they are causing harm, or potential harm. I have always been one to step in to correct another child when their behavior has crossed lines--in gentle ways, of course, but I do believe in the whole "it takes a village to raise a child" idea, and also believe that if more people worked together to help raise children, to gently guide them and be there for them, then they will grow up the better for it. But what would you have done in this situation? Last week I was pre-beach week shopping at Target with L., after we had dropped T. off at a birthday party nearby. On our way out of the store I saw a shopping cart parked near the snack bar. The mom was grabbing a handful of napkins from the counter. In the cart was a baby--maybe 17 months old, and in the back of the cart a boy, about two. A bigger sister, maybe L.'s age, was standing next to the cart. As soon as I walked past the cart the older girl punched the baby repeatedly on the arm, until she cried. The girl knew she had been seen, because when I made a face at her (a scrunched-up, you-should-know-better Mom look) she put her hand to her mouth. At that point the mom reappeared and the older girl ran to her. And told her that her BROTHER had punched his baby sister in the arm! To make matters worse, the mom scolded the poor boy for hitting his sister, and made threats to take away a promised treat. The older child watched all this with a smug look (oh, she was so smug), the baby cried on, and the poor falsely accused younger brother sat in the cart, looking confused. The whole way home I kicked myself for not intervening and letting the mother know what I had seen. But I was genuinely worried that she'd take my interference as a criticism of her parenting--somehow, and I was judging the parenting, I admit. It doesn't seem right to exact consequences on a child when you haven't witnessed the transgression with your own eyes. So I did the wimpy thing, and walked away. But I was haunted by the injustice, and by the idea that the girl had purposefully manipulated the situation to get her brother in trouble, at the expense of the poor baby. Was I right? Wrong? What would you have done?