https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3665046 (HT: Aaron Renn)
Since 1977, U.S. states have passed laws steadily raising the age for which a child must ride in a car safety seat. These laws significantly raise the cost of having a third child, as many regular-sized cars cannot fit three child seats in the back… We estimate that these laws prevented only 57 car crash fatalities of children nationwide in 2017. Simultaneously, they led to a permanent reduction of approximately 8,000 births in the same year, and 145,000 fewer births since 1980, with 90% of this decline being since 2000.
The median age for seat belts in U.S. states is now eight, and every single law change has been to increase the age, not reduce it. Enthusiasm for these laws has not been curbed by studies showing that child car seats are generally no more effective than seat belts in preventing death or serious injury for children above age two (e.g. Levitt 2008, Doyle and Levitt 2010). Part of this may be due to the perception that such mandates are virtually costless, beyond that of the car seats themselves (though for many families, even these may be burdensome).
Regarding the potential lives saved by car seats:
In the vast majority of specifications, we are unable to reject a null hypothesis of zero lives saved.
They controlled for tons of variables. Somebody asked me whether I think this is true. Having looked at the paper, and judging from personal experience, yes. Absolutely.
Edited to add: You can also add into the mix the EPA rules making it impossible to get the affordable 9 seat minibuses that are common throughout most of the rest of the world.