I am a baseball coach/manager. In my stepson's baseball league, another team has a child (these are pony league players - 13 & 14) who has some arm problems. I know he has had an MRI (know the MRI tech) and also that his doctor instructed him never to pitch again. The coach and parents are aware of this too - yet the coach still pitches him in games. Other parents discuss this problem, yet no one seems willing to step up and do something about this. Since I know the story, would it be ethical if I anonymously informed the league? There may be a potential liability issue at stake here too. This kid is going to ruin his arm before he gets to high school.
I am also trying to balance the confidentiality of the medical relationship vs. the kid's welfare. Should I even be considering this?
He will ruin his arm -- that's a major burden for a child who loves sports. The goal of saving his arm seems a lot more important than the other considerations you mention. But perhaps you don't actually have to make this choice. Instead of informing the league, you might just talk to the MRI technician (who already knows that you know, and is responsible for your knowing, the boy's condition). Urge the technician to tell the doctor that the boy is being pitched by his coach and that his parents apparently approve of this. (The technician can say that this information comes from someone else without saying that this someone else -- you -- knows about the severity of the boy's injury.) The doctor can then decide what to do -- e.g., telephone the boy and/or the parents or write them a stern letter, and, if all else fails, communicate her or his concerns to the league. The doctor best understands the boy's condition and can speak about it with the greatest authority.