One of my biggest bugbears is when I see adults, and specifically parents, out and about on their bike without a helmet. What kind of an example is that to set to your children?
Bizarrely, wearing a helmet is still not compulsory for under 16s. Regardless, it's still important to equipt your child with a suitable bike helmet (even if you'll have to put up with them moaning about helmet hair and being 'uncool')
You could try softening the blow to their cool points with this Youth Helmet Flames from ASDA (£11.20), which is still relatively cool for older kids.
Got younger children? Try these helmets on for size.