Mary Bell killed 2 toddlers, less than a year apart when she was 11. She was incorrectly diagnosed as a psychopath when in fact she was a broken, abused child who had been pimped out by her prostitute mother to various clients from the age of 2.
She was released and has lived a totally normal life, with a daughter, a partner and at least one grandchild. Albeit she's been very affected mentally and is apparently a meek and anxious woman.
Much as the desire for revenge is understandable, the purpose of the judicial system isn't just punishment, it's rehabilitation, and children who commit terrible crimes actually have a better chance of rehabilitation than adults.