Short version: Younger kids can definitely benefit from the training. In fact, it is safer for them to do this training than not to... here is why.
Long version: Kids are already engaging in this type of intensity in regular activities and sports but they are doing it in an "uncontrolled" environment. The workout takes the most intense portions and puts them in a "controlled" environment. In this environment athletes of all ages can strengthen their bodies in a way to prevent potential injuries in "uncontrolled" environments.
Properly learning to absorb and produce force in proper body alignment will significantly reduce the occurrence of injuries throughout an athletes career.
There is no research data confirming that proper weight lifting will damage growth plates. Some German studies have shown proper training to increase growth potential. Equal amount of stress is placed on an athlete during competitive play as during a proper training session.
I definitely recommend supervision and starting out with low weights or using the alternative exercises instead of the weights as a starting point. Both strategies are outlined within the manual.
Please let me know if this answered your questions and if you have any questions along the way. Keep me updated.