People are already allowed to eg feed their children unhealthy food etc. That's usually not classified as an externality.

Putting yourself and a child in a flying mechanical deathtrap and you're not wearing is an unnecessary risk to their life in the same way that leaving guns loaded and unlocked in a house is. These are differences in the magnitude of risk and a large enough difference in magnitude becomes a difference in kind.

That's mostly an argument for the law requiring children to be buckled in at most, perhaps.