Little kids are cute and all, but they can also be some of the biggest douchebags you'll ever meet. Some of it is because they don't know any better, and some of it is because they're putting into practice a terrible lesson about life that they've learned far too early.
This is especially obvious when they start being bullies. Not knowing how to share is one thing, but being purposefully mean is quite another. It's hard to stand by and watch something like that happen to your child when you're a parent. It's no wonder some choose to take matters into their own hands.
One Redditor decided to do exactly this when he witnessed a girl at the playground talking down to his son. People commended his quick thinking, with some sharing their own stories of dealing with mean kids. As an adult, it isn't always your place to reprimand children if they aren't yours. However, it can be worthwhile if it's wisely done.