Just a couple of thoughts here.
First, KPMarineopsdad is 100% right. We parents must let go. Anything that changes has to come from the PC. It is their responsibility. You will only create problems if you try to "fix it."
Second, welcome to the military. Yes, KP is the military. They belong to the regiment and the regiment's needs take priority over their desires. My daughter did not want to participate in Band and still doesn't. But it is important to the school. She has a needed skill.
Third, yes, it's a shame folks outside of Band Co. don't recognize the importance and difficulty of what the Band does. Welcome to life. Band Co. folks tend to stick together and keep to themselves according to my daughter. And she wouldn't trade them as company mates for anyone else! The internal support in Band Co. is fantastic!
Fourth, there is no reason your son or daughter can't be involved in any other activity or sport they want to. Band requirements will die down considerably after Beat Retreat season. Fall is busy, but it gets better. One of our daughter's roommates was on the crew team.
Fifth, if our daughter can succeed academically while doing Band and a varsity sport, it can certainly be done! I seem to remember her first letter home boasting that Band Co. actually had the highest percentage of students graduating on time of any company at KP. (That may have been company propaganda, but I think she quoted 80%.) Our daughter says that she did best academically while she had the added commitment of the sport. It forced her to stay focused.
OK, this turned out to be longer than I intended. But the best advice I can give is to step back, take a deep breath and see how it works out.
Good luck to all of you parents. It really isn't as bad as you fear. They will be fine and they will sort it out, just like SOOO many mids before them.