It evolves with age but in my experience kids really should not be throwing breaking balls while their growth plates are still developing. The other issue is control of a curve/slider which at ages 12-15 most guys don't have, you get ahead 0-2 and lose all that momentum and additional pitch count. My direction to the pitching coach was always 4 seam with a changeup focusing on same release/arm speed and a 2 seamer for movement. To me control was much more important at that age than speed or break.
On a personal note my son grew up with Max Meyer (Marlins) and I coached him for a few years and have followed his career closely. We were surprised he was picked so early by the Marlins at #3 who were quoted that his repeatable low impact delivery, great athlete, low mileage short-season arm made him less susceptable to injury!! He's just returning now from TJ but likely never threw a breaking ball before HS.
Emmet has some really good points the one thing I would echo is the simple fundamentals of throwing a ball which is why proper short/long toss is so important. Stretch and ice-two things kids hate to do.