There is one study that is typically used to show how much bicycle helmets decrease injury and increase rider safety. This study states that bicycle helmets can prevent 85% of head injuries, and 88% of brain injuries.
Apparently, the people who ran the study have said, the study to show reduction of head injuries did not involve a single collision with an automobile, or a single adult. Besides that, only the age 1-4 group showed an 85% reduction in head injuries, the 5-9 year range showed a reduction of 43%, and the 10-14 year range showed a reduction of 23%.
Better yet - the group has also admitted that when counting the head injuries of people not wearing helmets, they counted injuries to all parts of the body, whereas for people wearing helmets, they only counted head injuries - which means that the injury ratio between helmeted and non-helmeted people is completely skewed.
Besides the obvious validity problems with the study, the fact that most major injuries (and especially head injuries) and nearly every single death on a bicycle is a result of a collision with an automobile makes the study basically irrelevant, except that it suggests that your kids may benefit from a helmet by getting fewer scratches on the top of their head while riding a bike alone away from traffic.