Why would it be so hard to drop the car seat requirement? Taxis are exempt from them, the security benefits are questionable already (more so once you factor that an autonomous car would probably have a different cabin and safety features), buses and trains don't need them...

Not to mention that a society that gets away from being car centric would also grant more independency to children from their parents. In Japan, kids are used to go to school by themselves at age 5. In Berlin I see kids seemlingly from elementary school going around alone in the U-Bahn. Looks like parents not having to be kids' chauffeurs would be a net gain.