"There is no rule or requirement about helmets, but they have become the absolute norm with kids. Helmets are cool!
Children might fall down and hit their head, but it's more likely that someone else might be skiing out of control and collide with them. Helmets will also keep a child's head warm and they're a good place for rad stickers.
Parents should role model safety (as well as being cool) by wearing a helmet too. Alas, most of the cartoons show adults without helmets - that was the illustrator's fault."