Regarding the age of a youngster participating in an intervention, I think it is a good safe way to get those emotions out and knowing you are not the only one thinking, feeling or seeing it.
For me, It was being a part of something that I was already a part of. I believe I would have been upset being left out and unheard. Kids want to be heard too.
It can get very ugly so I believe a controlled intervention with a specialist would be best if children are to attend.