Heads injuries are the most common cause of fatal injury in children. Bikes are great fun and a good way for children to keep fit. Unfortunately, an injury is all about physics. I have yet to see a child run into something and have a significant head injury. Bicycles, however, allow a young person to gain enough momentum to do real damage even if another vehicle is not involved. While it can be difficult to persuade young people to wear helmets, they are the must-have accessory for anyone who likes their brain or their face. Road rash on the face is not a good look and helmets do a decent job of protecting the face from being badly grazed in a fall from a bike.
I emphasise the facial injury as much as the head injury prevention as it often means more to the young person involved.