Some tennis coaches know nothing about special conditioning for tennis. Some coaches just do not want to send their players (clients) to a specialized fitness coach, because they want to get as much money as possible. That's why these coaches try to use on-court fitness (with very little understanding) and tell to their players that they (coaches) are able to develop all necessary skills. I already wrote that not many players warm-up and cool down in right way. As concerns how many hours court time a week: any organism has its own limits. If a kid trains in right way, using periodization and cycles with a good coach, 12 hours a week on the court till 15 yo, is more than enough + off court fitness and tournaments.
All these talks about 25-30 hours a week for kids come from:
1. incompetent coaches, who never studied Sports Science,
2. academies and coaches who love money more than their players. Parents pay for a coach's time on the court, not for the final results. It is very simple.
Just look at ITF recommendations, I posted them in my article: http://wp.me/p2iyrR-2W