IMO the single biggest thing that any athlete's parent or coach could do for them is to determine if they are or are not quad dominant. Quad dominant kids' sports futures will always die a long slow death... Pretty much by 16u the condition is terminal. However I believe if caught early enough, a targeted training program can be developed to correct or minimize the long term effects.
If the kid is not quad dominant and has good functioning glutes, ballistic workouts that target glutes and explosive movements will offer the biggest gains as the benefits go directly toward speed and first step performance. This relates to pitching in that by 14/16U, drive mechanics and explosiveness on the mound become the biggest power sources for pitchers (where as at 14u and before, geometrical attributes like tall/long arms can be more dominant). Improving thrust from the middle will result in big gains in pitching speed. It will also improve drive efficiency which means that kids can back off the throttle slightly and reign in control.
Working the glutes is not about developing big derrieres at all... Strength is not the key, the goal is to improve the entire posterior drive linkage which extends all the way down to the feet. Explosive exercises that load the legs down to the balls of the feet can produce big gains. I'm a fan of depth jumps using plyo boxes, but it's important to target optimal starting heights as too little/too much will slow progress.