As young players start practicing early in the season, it is important that they get sound instruction from the first day. In the beginning of the season, they are excited and often very enthusiastic, so it should be emphasized that they should not overdo it. Children should not throw overly hard or they will not be able to throw the next day or the day after. Coaches need to temper the enthusiasm and reinforce the proper techniques immediately. Also, coaches need to limit the amount of throwing at the start of the season.
Some leagues limit pitching by counting innings.
Because innings can vary widely in the number of pitches thrown, a far better limit is the actual number of pitches thrown. As a pitcher matures and builds up arm strength, the number of pitches thrown can gradually increase. It is my opinion that an improperly thrown breaking ball causes problems, not only in young arms but also in seasoned arms. A properly thrown curveball is no more damaging to young arms than a properly thrown fastball. The trouble that most young players face with throwing the curveball is that they radically change the mechanics of their usual throw often dropping the arm sideways to try to spin the ball so it curves sideways.