I was at a park concert once, the kind where everyone is sitting, and there was a large family in front of me. One of the kids (around 12 I would guess, old enough in any case) kept standing up, roaming around, and generally blocking the view of the people behind him, even though he wasn't watching the show (he was literally munching on snacks and facing back in the crowd). I wouldn't have cared if he was excited, dancing, enjoying the show, but that was just not the case.
I asked him if he could sit down so I could see. His dad immediately got in my face and demanded that I apologize to his son. He didn't leave until the police got involved.
But fundamentally, keeping control Iverson a kid is on parent and I don't think about bumping kids enough to form a sentence. I just expect parents to tell them cause it is their job and in this story, the parent in fact intervened.
People are terrified of saying anything because of the likely reaction of the kids' parents. The "it takes a village" mindset is gone. If someone's kid runs into you and you so much as say "excuse me" to them, you risk the parents getting in your face, holding up their cellphone camera, yelling "You don't talk to my kids. Go mind your damn business, Karen!" Not worth it--so bad behavior in public goes uncorrected.
Even when kids aren't involved, people's reaction more and more to public criticism/correction is to get belligerent and tell you to mind your business.