I am somewhat skepticle about "breaking pitches" hurting young kids arms. From my own experience with my son, the only time his arm hurts is when he throws long innings/ long games and has to rely on his fastball to get him out of trouble. In fact, I have had games where my son went more to his breaking pitch because as he says- "my arm was getting sore from throwing too many fastballs". My son says he can throw curveballs all day without soreness. Maybe he just knows how to properly throw them.
I have read studies where the most damage that happens in the joints of arms is from over-stress from throwing too many "fastball" pitches. From my experience, this truly seems to be the case! Almost every kid who I coached, when he threw hard in a game almost always complained of soreness or tenderness in his arm- and they weren't throwing hardly any breaking pitches if any at all! Last year we had one kid who came into a game and was throwing real hard, and the next day he was still in pain and went to the doctor and was told to lay off pitching for a month, that he had an overtressed ligament in his arm by his growth plate. He threw nothing but fastballs in that game.
Truly, I can't understand how curveballs are more dangerous to throw. I have slowed down footage of my son throwing and from a mechanical standpoint nothing changes between his fastball and curveball except for the placement of the hand and the release point. Both the shoulder and elbow still have the same motion through the delivery.
I am still not convinced that throwing breaking balls is more dangerous than a good hard fastball. the jury is still out for me I guess.