Take a look at the popn pyramid:

44-44 group about 120 million

0-4  group about 80 million.

One might say let the 0-4 group drop to 60 million then every couple start having 2 kids. However as I linked before, china is already in the social econ stage that people will have less than 2 kids/couple voluntarily. It isnt cheap to raise 2 kids in urban area, the more urbanised china is, the lower the fertility rate.