NFL is not the only league facing litigation. Any of those leagues or institutions you mentioned are open to litigation and anybody can bring a case against them. There already was a class-action lawsuit against the NCAA for negligence and lack of enforcement of safety rules in college football. Don't know if they settled or if it's still going on. But this isn't new, you just may have not heard of it.
If your child suffered head injuries playing Pee Wee football, and there was no medical waiver signed, and the Pee Wee coach allowed the player to play even if a doctor hasn't cleared them, then you have a case against the coach and the league (as an employer of the coach). You have to back it up with medical history. This is unlikely because those leagues are not as physical, plus there is the issue proving long-term effects in a sport where contact is already limited due to age. Let's be honest, those kids, for the most, part are not top-class athletes trying to physically dominate each other.
In the case of the NFL players, a lot of them can specifically trace their symptoms and injuries to their playing days in the NFL, not the NCAA or junior leagues. And this is backed up by the medical documents which teams kept on players which show exactly when they sustained injuries and the treatment they received. That's why they are suing the NFL, and not the NCAA or their high school. One of their main arguments is that NFL , their employer (not the NCAA or college), did not inform them of the long term risks of their injuries and in some cases even allowed them to continue playing. Hence negligence. It's hard to prove tho which is why the case has been going on for so long, plus NFl has crazy money and can drag it out.