I see all the parents here talking up their kids that are in martial arts. I think they are a great past time for the kids, teaches discipline, keeps them active and something outside of school to look for, and yes, helps with self esteem and maybe a bit of self defense.
But, your kids should not rely on that to be safe, it could seriously backfire if they think they can take someone because they have "training". I've seen it a bunch of times when we have a bit of a fun scrap at work. (We get to do some self defense stuff from time to time.) Mr. "Black Belt" gets surprised when the farm kid doesn't play crouching tiger and gets walked on just with straight force.
However, there is some real fighting training that mean business. The choreographed "karate" stuff I learned as a kid really isn't something I would rely on.
Now for the video, I can see why they ganged up, not one of them could throw a real punch, and I saw only one actually decent kick. Guaranteed that kid could have taken any of them on 1-1. He did the best he could, got up, found an opening and got out. The only other thing he could have done is not got himself in that situation in the first place.