Earlier quoted context omitted.
I could support kids learning boxing if it was limited to body blows. With what we are learning about concussion, it seems to be more and more understood that blows to the head are never OK. Maybe you still need to teach defense against head blows, but they should probably not be allowed in competition. Getting battered in the head until you cannot stand up or respond should be right out.
There are martial arts where even in competition, concussion is relatively rare. E.g. grappling sports like Brazilian Jiu-Jitsu don't usually involve impacts of that intensity, to the head or otherwise.
And soccer has the most injuries of any sport, according to an ER doc (who treated me for a football injury).