Welcome to the HSBBW BaseballDad1. Good screen name! Ahhh, the elusive 10 more miles per hour. If there were a guaranteed method for adding 10 mph it would be so well know that no one would have to ask.
The first thing I would suggest is for you to take your sons to a qualified pitching instructor. You don’t have to break the bank here. A couple of times a month should keep your sons on track. Their mechanics need to be correct prior to expecting any velocity gains. If you attend the lessons and LISTEN to what the pitching instructor is teaching, you can continue those lessons at home and work out an “exercise schedule” for each son. Their pitching instructor should be more than willing to help you devise a plan. I made a workout schedule (and some homemade gimmicks) for my son that included thera bands, rice bucket, resistance exercises, light weights, running, and long toss. Even with my “homespun” method of training I was able to significantly improve his velocity ---- but then again he may have thrown harder if I had left him alone.
Fungo