You take your little one to ski school and ask if you should get them a helmet. Most ski instructors are instructed to say something vague like, "I wear one but it's a personal choice."
The fear is that if a child is hurt with a helmet on whether because their neck gets caught in the strap or it doesn't protect their head properly, the parent will sue the resort. So employees are told not to make recommendations.
Yet a helmet is required at every freestyle and big mountain event. That should speak volumes. Kids love to play in trees, to take jumps and ski without turning. It makes sense to keep their heads protected.
a 10-year-old Texas girl died at Ski Santa Fe Dec. 30, 2008, after she fell, slid into a tree and hit her head while skiing with her family. Patrol tried to revive her but she was pronounced dead at the scene. She was not wearing a helmet.
LidsOnKids.org says that wearing a helmet is "a smart idea." Yet, they say you would need to ski or ride as if you weren't wearing one. Apparently, we tend to be less careful and overconfident in a helmet; skiing in areas we might not otherwise (trees) or doing things we might have avoided (hucking cliffs).
There are other drawbacks to helmets as well - restricted vision and no guarantee that it will protect your head from impact when you're flying down a hill at 25mph (most helmets are rated for drops around 15mph).
The bottom line seems to point to personal preference. But since kids ski with abandon with or without a lid and they're often too young to be aware that a helmet might give them a false sense of security, it might be worth it to study those companies making junior helmets and decide for yourself.