So many times, I see kids with helmets that are basically dangling on top of the head with the strap loose. I doubt such a set up would do much good to prevent an injury, particularly at a high speed.
So again, remind your kids who longboard (or bike, or roller blade, etc.) not only to wear those helmets, but to wear them properly. You'll get the dismissive shrug of the shoulders from them. But then you might do what I did and tell them about the study last week that found a heightened risk of head injury. They'll pay attention.