There is research (somewhere, cue Hugenot) which suggests that children under 12 cannot judge car speeds properly, which means children crossing roads and/or cycling on roads are at a disadvantage. When my daughter began cycling on roads (after the superb free course from Southwark), I researched helmet pros and cons. I came down on the side of no helmet for various reasons, including the fact that those who wear helmets and high vis jackets seem to think that they are invincible.