An excellent question RinMyP. We can all play the arm chair quarterback and hind sight 20/20 game to come up with the pros and cons of any given situation. If this ride had gone bad, then I would suspect as you elude that many of us would jump to criticize the dad. However, I think after watching the video again, that dad had the situation under control. I expect that this was not the first try at this ride and that the youngster has had a previous shot at learning the technique. As soon as dad gave him the signal, the kids instantly applied a correct amount of throttle twist to maintain speed. It seemed well played to me.
To bolster your point though, many people will criticize the dad simply for letting the child be on the bike let alone be in front and with no regard to holding the bars. Some of us will never win in these discussions no matter what risk avoidance measures are in place.
I still stand with the dad in this one and lift my drink in his honor for teaching the boy something in life that has no amount monetary value but instead is priceless.