You can take photos and video in public spaces as long as you're not harrassing anyone. Just don't share/publish them with anyone other than the Gardai if they are idenifiable from those photos and videos without getting permission from the individuals or, in this case as they are minors, their parents.
If you video them breaking the law you can bring it to the Gardai.
We had some egg throwing kids a couple of years ago - a small gang of 6 or 7, 12/13 year olds. They were caught red handed by myself and my husband who marched over to their house to talk to the parents - they weren't home. We were going to return when, about 15 minutes later, all of the young boys turned up on our doorstep, heads down and scared we were going to tell their parents when they got home... which we were. They offered to clean up the egg mess, mow our lawn and any other jobs we needed doing - suddenly they were boy scouts but they were genuinely repentant because I think they were more scared of their parents than of us. We never told their parents and we haven't seen them since - they're still keeping their heads down.