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.