All longboarders wear helmets.
The only longboarders who do not wear helmets are beginners. You'll see most skateboarders at skateparks do not have helmets on, this is because of the type of skating they are doing. Skateparks are a controlled environment. Most skaters know their local skatepark very well, and any falls are controlled or expected. Longboarding is different. They are on roads, footpaths, bike tracks. there are dogs, children, rocks, cracks, gutters. The fall the longboarder takes is uncontrolled and unexpected. Longboarders also travel a lot faster. This means longboarders must wear helmets. If your child refuses or thinks they are uncool, then just show them videos and photos of all the pros wearing helmets.
If they skate with friends or go to organised events like Sydney City Bomb Squad, they will not be able to participate without a helmet.
Most young children are use to helmets as they are compulsory for biking. They can use their bike helmet, many skate helmets are bike rated.
Young children (3yr to 12yrs) MUST wear a helmet for longbaording or skateboard. In the park on the driveway.
With teenagers it is harder to enforce if they are skateboarding in the skatepark. Bribery usually works best.