Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

The decision as to whether a child ought to wear a helmet or not, even when making the decision for them, is based on the same assessment criteria as an adult. You logic makes a leap from children not being able to make their own risk assessment to de facto having to wear a helmet. Your paradigm ignores the risk assessment stage being made by anyone, let alone a responsible adult.

The decision is being made for them by a responsible adult, who has the duty to weigh up the risk factors and balance them against the real and perceived benefits.

Your argument doesn't really need derailing as it's not up to much in the first place.

Granted, I'm making helmets the de-facto standard, but I just think why wouldn't you? I have my reasons for not wearing one, but they are not related to safety, and in that sense it's not a particularly evidence-based judgement. If I were learning to cycle I'd wear one, and I'd encourage others to do so too.