Well the replies are a breath of fresh air.
Personally I feel that the wearing of helmets DOES prevent serious head injury in case of a fall, collision or head impact. If the wearing a helmets becomes compulsory for cyclists, it should also be compulsory for drivers, pedestrians, office workers and most definitely children playing!
Like many folk my age, we grew up on our bikes... my brother and I wanted to be stunt men when we grew up so spent lots of time training, building ramps (big bro jumped seven children on his Grifter!), doing big skids, even deliberately crashing spectacularly. He wont let his kids go out on their bikes without their helmets on, and in light of this they seldom go out on their bikes, which is a shame.
I'm totally against it being compulsory for kids... as a child is your job to take risks and occasionally injure yourself.
Wrapping them in cotton wool and discouraging them from risky play does children no good at all.