The gym is ultimately responsible for the actions of its employees. They train coaches and set expectations. Not every coach they hire may live up to those expectations, but how the gym handles a coach that violates their rules says a lot about the type of business the gym runs.
I don't believe NO ONE other than the coach knew the athlete was underage. Kids talk. They know how old everyone is. Not all parents are clueless about cheer rules. The office staff or owner or someone handles birth certificates and usasf registration. Someone else knew and looked the other way.
Ignorance is not an excuse. A gym owner takes a chance when they put someone else in charge of paperwork, creating teams, coaching, etc, but it is THEIR name on the door at the end of the day. They should be protecting it as hard as they can.