I value more is not having children with lifelong brain damage

First, you were talking about enforcing helmets as something that should apply to everyone, now you are talking about kids. Meaning, you just moved the goal posts.

Second, and to go back to the point, if you are concerned about children's safety we need to have (a) proper bicycling infrastructure and (b) less and smaller cars. In the US, I'd venture there are more kids dying because they are being run over by those ridiculous trucks than because of bike accidents.

Third, at least here in Germany the law is quite simple: kids until age 12 must ride on the sidewalk. That basically (a) forces them to go slow due to pedestrians and (b) avoids the whole issue of having to deal with traffic. Helmets are not mandatory, but absolutely normal.