Agreed. Just an observation. I don't think falling on your back is common (except for
@scott43 , apparently!)
I've never seen a fall backwards to your back, but I have witnessed (on more than one occasion), the ol' over the handlebars, front flip, somersault, land on the back.
As for the video, I see nothing wrong with it. As Monique pointed out, it looks like the kid is pumping off a couple of the rollers, and leaning into the berms. Good teaching opportunity, IMO. The dad (assuming it's the father), isn't going that fast. Seems like if there was a fall, it would be no worse than if the kid was on his own bike. I agree with everyone else, the kid seats on a bike or kid in a backpack makes me significantly more nervous. The idea of being strapped in during a crash seems much scarier and more dangerous than being on a trail-a-bike.