Kids are special little humans, aren't they? Running around all sticky and snotty, asking you a billion questions until your head pops. With your own kids you can do whatever you wish. You can discipline them. You can tell them to leave you alone. You can praise them when don't deserve it. You can do and say anything. With other people's kids you don't have that range, though, nor should you. However, and even though other people's kids aren't mine to discipline, there are times when I'll speak up and definitely say something about other people's kids.
Whether they want to or not, parents are always modeling behaviors to their children. Our kids are constantly watching us and taking mental notes of how we act and what we say. When we stand up for and to others, our kids know. When we are silent in times of injustice, especially when that injustice is towards our children, we aren't only doing our children a disservice, we are teaching them it's OK when someone mistreats them. We are essentially telling our children they should not stand up for themselves (or someone else) because that may hurt someone's feelings. We are showing them that they should silently accept whatever it is that is happening to them.
Similarly, when we remain silent when we see a child being good, we aren't encouraging confidence and benevolence. When we tell other parents how great their kids are, we foster pride. When parents take pride in their most precious work — raising their children — we all win.