You know, there's been times when I've asked a child in front of the parent who was screaming at them or yanking on their arm, if they were okay. It tells that parent that people are watching. If they would ever get nasty, I'd have no hesitation to call 911.
The first camping trip with the Lovebug, there was a woman who screamed at her kids nonstop and hit them. I told the campground manager and called social services on Monday. Sheesh, you have to do something.
I was reminded last weekend about an incident I had a few years ago. While traveling on a busy highway, as a passenger in a small car in the middle of the afternoon, I thought I saw someone lying in a ditch. While my husband dismissed it, I insisted we turn around and take a look. There was a man there, he had smashed his car two days earlier and had crawled across fields for two days with a compound fracture. He had been in that ditch since the night before. He'd lost a lot of blood and was very sick by the time I saw him. He turned out alright but perhaps mayn't if someone hadn't seen him that day and stopped.
If your instincts say, "Act!" then do it.
Great video, thanks for posting it! It would be good for all parents to see this. If this girl had been yelling "Stranger, help!" Perhaps it would have spurred someone to have acted differently?