I can't really say without knowing what they're saying or what he did, and you also have to take into consideration the legislation for security guards in the specific country (around here they're not allowed to directly act upon you without the presence of police officers, if you did nothing that requires their intervention (i.e.: just walked out of the store and they think you look suspicious)). Could anyone offer a translation?
From a first glance though, no, he did not act right. You do not handcuff a 12 year old and you do not throw him to the ground and place a knee on him and, thus, half your body weight. Especially the way he acted; that didn't scare him, it just made him more rebellious. What did the kid do? Throw rocks at your or just walked by saying "fuck da police"?
And as for the "kid had it coming. he was being aggressive"... really? What would you have done if it was a 14 year old? Open fire just to be sure he doesn't get too close and overpowers you?