What ChrisL said. No point telling peeps to wear their helmets. Even when you get down to the couple or so kids that actually wear their helmets, a great majority of those aren't wearing it properly. Wearing a helmet properly doesn't 'just happen'. I highly doubt it would 'just happen' for kids whos parents have no clue either.
The only real time I see kids with properly adjusted helmets is when they're with their parents who are obviously "real" cyclists.