In that case you can't oppose China's one-child policy on rational grounds. Even if you had a solution, it would not work because if China was able to import enough ressources, have enough room etc for an increasing population other countries and people would suffer.
A rich country with a high demand for something causes the prices to increase greatly.
You'd have to chose the well being of one nation over others.

But you can use the very same argument the other way around for other countries.
Germany, for example has around 80 million people.
The birthrates are low so the amount of people decreases. That, too, is a problem.