Here is an idea. Build a brand new building in the corner of the adult prison's rec. yard, fence it off.
The little punks that want to carry out acts of violent like this, you try them as adults.
They go to that new building in the rec. yard until they turn 18. On their 18th birthday you wake their asses up and say "Happy Birthday!", today, you go next door. Your rec. break starts when they all come out, you will be assigned your new cell after break. And they finish their sentence off.