Earlier quoted context omitted.
One thing that bugs me about that video - are they really going 60-70 mph on normal city streets?
Also, that kid in the back seat should at least be in a booster seat. The seat belt is way too high on him.
It is identically positioned on both him and the teenage girl sitting next to him. Which should hint that it is correctly positioned.
But to get specific, a seatbelt is incorrectly positioned when the lap strap goes across a child's belly OR the shoulder strap doesn't cross their collar bone. Like this: http://i.imgur.com/Zu9pmBv.jpg
In this video the boy (or girl) didn't violate either of those guidelines, therefore he does not require a booster seat.
I wouldn't go much smaller than he is without one however.