According to a recent study of 500 children from ages 4 to 18, more than 70% of kids are wearing bike helmets today. The bad news is a lot of kids are wearing them wrong. That same study in the Journal of Pediatrics showed only 4% were wearing them correctly.
Kids tend to wear the helmet either too high on the forehead or the strap doesn't fit around the ears in a "v" shape like it should. Also, the helmet may slide too easily forward or backward on the head. So, the helmets aren't protecting the front of the brain which is the most common site of impact in bicycle head injuries.
The study suggests that pediatricians encourage kids to bring their helmets when they come in so that helmet-fitting can be part of a child's regular check-up.