What is this, amateur hour? You WANT to kill them? The kids who were forced into this ring will have to live with that for the rest of their days. Prison. Isolation ward for their protection. And no chance in hell of parole. Show them that hell can be both a lack of people AND it can be other people.
If the death penalty must be applied, give those kids some rocks and give them the option of going to town on their tormentors.
I prefer the former punishment. Give them a fair trial like everyone else, and ensure that they will never have any meaning in anyone else's lives.