Even at a primarily western barn I used to ride at as a child and younger teenager, the rule was "helmets MUST be worn, even if you are just sitting on the horse". I know a lot of the kids just took their helmets off in the woods and hung them on a tree when they went trail riding. I never did. My trainer made even adults sign a waiver to not wear a helmet, and no one under 18 was allowed to do so even with a parent offering to sign.
It's a choice for adults. I wish everyone chose to wear one, because helmets have saved my life before. And history has proven even the most experienced riders can have a deadly fall. But for children, I don't think it should be. Children, I hate to say this, cannot really comprehend the real seriousness of their decision to wear or not wear a helmet. They can understand they "could die", but not really fully grasp it. But even though I wore one, that did not really sink in until I was a bit older. Especially younger children should always wear a helmet because they are young children who are not experienced equestrians and the likelihood of a fall, whatever the cause may be, is greater. I just feel like you don't let your children ride in the car without a seatbelt, even if they don't want to, because you know it is in their best interest: the helmet situation is the same concept.
As for that story, the helmet was not the only thing wrong. That was a stud, and I heard, a rather temperamental one. A child had no business even around him. How very sad that her life was cut short because of ignorance on her parents part. :( I saw the story earlier, and it's been on my mind all day...
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers