When I worked with emotionally disturbed kids, they trained us that you only escalate the hold if they won't be talked down and if they continue to try and hurt you. In this case, since the child was kicking at the security guard's knee, he probably would have been justified in taking him to the ground and holding him there until he was calm. Of course it's a little hard to tell from the video what the two were saying, but it doesn't matter how old or small the kid is, if he is behaving in an unsafe manner the security guard should have done what was necessary.