I read on HN a pretty convincing essay that if a car-seat was only required while under a year, and a booster while under 2, there would be something like 10% more births in the U.S. every year.

There are always those families (especially among the religious) that would have larger families, with not obvious things that limit them. Knowing nothing about Japan, but living in the U.S. where birthrates are also falling, I would think state effort should be put into this.