You're going to the very extreme end of what he meant by "loose"...

You won't gain additional momentum from a loose belt. But a loose belt may give your body the chance to slip out. The impulse you feel from a tight belt will be no less than the impulse you feel from a loose one.

I'd rather redirect my momentum into soft webbing spread across my lap, than into my head impacting an overhead bin.

Pricesely this, yes. And for the kids, an average trip to school takes like 5-10 minutes, hassling with three helmets (mine and of two kids) takes almost as long. And then I have to carry three helmets if I want to drop by to the supermarket after getting them to school.

The bicycle seats for kids are designed in such a way that the only real danger of a head being hit is in the frontal collision, or in a really forceful side one. The design of Dutch streets makes the chance of either happening small enough that the helmets are widely seen as not worth it.

Also it's just not really a big danger. There's no epidemic of brain injury due to this.

Part of the reason is that because bike usage is so common and the infrastructure is very separate (bike lanes everywhere, no mixed traffic on eg roundabouts), and bikes almost always have right of way and are not to blame in an accident, that drivers are very aware of them. I noticed in countries like Ireland with much less bikes that bike lanes sometimes just end in the middle of a high speed roundabout. And most drivers there have no clue which is in part because until 10 years ago you could just buy a driver's license at the post office. It was a "learner's permit" but it was normal practice to just go and drive.

So yeah in Ireland I wouldn't even ride a bike with a helmet. In Holland I'm much safer even without one.

This really isn’t a similar comparison, Dutch infrastructure is built to prioritize making cycling safe at every decision point.

Whereas you’d be someone with a death wish to be riding without a helmet in most places in the US.

Actually that seems very Japanese in a weird way - they might be trusting themselves and everyone else to follow rules precisely, thus avoiding all the accidents and obviating the need for seatbelts.