The cause and effect is not so clear. China's wealth has also risen substantially, and with it comes fertility decline. E.g. India reached below replacement fertility a couple of years ago without it.
I think it's likely the one child policy contributed a bit, but a substantial part of the decline is clearly also due to economic development.
I think it's likely the one child policy contributed a bit, but a substantial part of the decline is clearly also due to economic development.