i ll just mention that a lot of the best horse polo players compete at a young age, 15 yo or even less ! and you can die trampled by your own horse.
i think also organizer themselves shouldnt be critisized harshly for a decision they had to take without guidance and rather hopefully this talk sets a new standard.
also if i was orga i would make you sign a form as their legal representant that you are aware of any danger your under aged sons are exposed , and happy with it.
i disagree strongly on your previous statment that injuries only occurs with violent plays
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a ninja ball can take out a heavy player using clipless shoes making him topple on your youngest kid, even while everyone being careful.
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if your kids are actually good and pacing the game up, the competitive nature of a tournament might push them to get into more dangerous situation than just pick ups ( but the nature of the tournament dictate this ie: “chill” tournament vs national qualifier)