Don't worry. Young people grow and become stronger almost by dint of getting older. Add in a little progressive resistance exercise and they WILL gain muscle and strength. It just comes a little easier to some kids than to others.
That said, I am NOT a big proponent of crazy volume or high-frequency training like some of the trainers you referenced are. IMO, one of the best things you can do for your kids is NOT to develop training angst.
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