Quite. So why are motorbike helmets, car seats and seatbelts mandatory then?
Because lots of people were killed or seriously injured in car and motorbike crashes, which was reduced by mandatory helmet/seat/seatbelt laws. There were no real downsides to implementing these laws. These deaths and injuries had an economic cost to society.
Very few people are killed or seriously injured while riding bikes, in ways which would be prevented by a mandatory helmet law. In countries with mandatory helmet laws, rates of cycling have decreased. The cost of the decreased exercise outweighs the cost of the deaths and injuries.
Those kids riding their bikes without helmets were at less risk than kids sat at home playing on their XBox. Healthy kids outside are much safer and healthier than fat kids inside, regardless of helmet use. Having fat kids is more “criminal” than letting them ride bikes helmetless.