For what it is worth, I will tell you what I did with my son and his teammates. Every kid pitched in practices and games. When they were 13 years old, they did flat ground work during every practice, and I also had them do breaking ball drills. They were not allowed to throw any breaking balls in games.
When they were 14, we continued with the breadking ball drills at practices, then they were not allowed to throw breaking balls in games until the middle of the season, approximately the last week of June.
When I allowed them to throw breaking balls, they were only allowed to do so on two counts (0-1 count and 1-2 count). This limited the amount of breaking balls they threw in games. It also required that they threw first pitch strikes and that they learned to throw fastballs out of the zone on 0-2 counts.
It worked for my kids at that age. The rules were adhered to even in big games and in the post season. The team was a very successful team.