I think you need to approach this like any investment....
Musicians are flighty. Teenagers are flighty. Combine the two, and you have enough potential material for a reality show, soap opera, sitcom, and pro wrestling plot. Do you really want to invest in that as the core of your business?
Also consider what other opportunity costs are involved. Your share of $7100+ could buy your son a nice rig, a years worth of lessons, his own promo materials, etc. which may parlay themselves into a gig that's profitable for him, as well as facilitating his development.
I also don't think that you're fully exploiting what the teenagers have. As adults, we have money and knowledge. But the kids have more time, energy, and friends. They should use that to attain knowledge and resources. You can do so much DIY now a days that it would be a shame to not let them mess around while they have the time and energy to do it. It's not like they have kids to support or a mortgage to pay, so let them work on it. Rather than spending $3K on a video, why not spend a few hundred on some video cameras and free editing software and let them play around with it? Instead of $3K on an EP, how about a few hundred on microphones and an interface and let them learn the process? Instead of spending a bundle on photos, they could find a friend at school who is a budding photographer to help. You never know, they may actually have more fun doing it that way. And in the end, they'll learn another valuable skill that they could use even if they quit music. And because they put more effort into it, they may appreciate it more. I never had parents involved in bands, and it made me more resourceful and knowledgeable because of that. Kids have time and energy, and they should fully exploit that. Time and energy help compensate for mistakes, and mistakes are a part of learning.
At that age, it's all about learning. IMO, there's no better time for learning than your teens/early adulthood. It's where opportunity, energy, and available time intersect the most often.