A theory that at least is consistent with the observed correlation seems vastly superior to a midbrow dismissal that doesn't. Your "raising kids is hard" theory would explain why people don't have a third child, but raising kids is hard universally. What was observed was that a third child was delayed for longer (even indefinitely) in states with higher age thresholds for mandatory car seats (even when controlling for demographics).

Their causal explanation relies on two additional observations that seem pretty hard to explain by other theories: the effect disappears for single-parent and carless households.