you do have the right to make nearly every decision regarding your family.
and if you really don't want your kid to wear a helmet, then either a.dont let them snowboard or b.dont bring them to resorts. i can't think of a single responsible parent who wouldn't encourage their child to wear a helmet anyway, and now when they say "i dont want to, its not cool" you say "it doesnt matter because its against the law not to" in the same way they must wear a seatbelt, in the same way they must be a certain height to ride a roller coaster, etc.
Never Summer Evo 153/Restricted ReFlex Cartel
Gnu Danny Kass 156 BTX