I agree with Mississippi.
If anyone ever hit/kicked/slapped my kid without provocation, I would be on them so fast that they wouldn't have a clue what hit them. There would be no harsh words, no assessing the situation, no nothing. Sure, the kid didn't get kicked hard at all, but that would be inexcusable in my eyes. I am extremely protective of my family. Anyone hurts or tries to hurt them immediately has to deal with me.
Understand though: I am not a violent person. And I hope the above statements don't make anyone here think that I am. All my life I've tried to follow a personal mantra of "do no harm." Even when I used to be a bouncer. But if someone tried to hurt anyone that I loved, I don't believe that I'd be able to contain myself.
As for the video, it appears that the guy attacked the other guy with Downs before he realized that he had it. Sure, the kid may have surprised him by running up on him like that, but kids don't always realize what they are doing, be it an innocent issue or otherwise. In no way to I blame the child for what happened. The whole scenario seems to be a huge misunderstanding. Hopefully no one really got hurt.