Today there are many narrow car seat options on the market, so 3 across seating is possible in full size sedans, but not compact cars. A European company makes a 3-across and 4-across car seat, but it's illegal in the US by accident.
The cost of daycare and education is so immensely larger than the cost of a car, that I don't think cost of a car is a factor. The important consideration with having a child, or another, is how could you possibly afford the daycare and later school.
The car seat related drop predates high daycare costs. In some places, high daycare costs aren't the primary concern: it's infant daycare waiting lists longer than 9 months.
Grandparents used to look after young children. Maybe we should incentivize that structure again? I'm open to other ideas.
Right now, childcare can't be affordable without the workers being paid exploitative low amounts. We should fix that, that's exactly the sort of problem that having a society is for.
Lead in the drinking water is okay. Last resort human antibiotics in the farmed animals are OK.
Seems like school shootings are OK.
I wouldn't use "illegal in the US" as something necessarily negative.
The full bench car seat is significantly safer than single seats. But it cannot be tested on the testing sleds in North America, and the wording of the regulations implicitly forbids multi-child seats anyway in many ways. Unfortunately, fixing the regulations is probably ~10 years and ~20 million dollars in lobbying.