Brain injuries or even death from bike or skateboard injuries are devastating. They can often be avoided.
The best thing you can do is to get on the helmet kick. Always remind kids about their helmet and make sure you start them at a young age. Make using a helmet a natural part of their bike or skateboard.
The best way to start is by having the child pick the helmet you get them. This way they have ownership over it. They picked it so it is best for them.
A major part of keeping kids on the helmet kick is to not be a hypocrite. Wear your own helmet when you do something where a helmet should be worn.
Show them. Watch any professional bike racer or pro skateboarder and point out their helmet. Bring them to a professional rally and show them, "it's what the professionals do".
The bottom line should always be, no helmet, no bike. Stick to this mantra when dealing with safety. You catch them without their helmet and no bike or skateboard. Take them away for a bit.