This is probably preaching to the converted but I've just found out that my best friend's nine year old was in a collision yesterday with a car while riding a bike. Unfortunately his bare head was used as a stopper.
He's now in hospital with a shattered wrist, which has had to be pinned. His head is cut open and there is talk of plastic surgery (not sure why yet).
This happened on a campsite where the speed limit is 5mph and on fairly flat ground.
My friend's child was lent the bike by his friend and found out to his cost that the brakes didn't work.
My friends are not outdoor types and don't take their children cycling, so wouldn't think to check a bike over or see if anything was wrong before hand. So it seems that it is always best to wear a helmet even when cycling 'just for a moment'.