Also, I think standard "trot out a toddler" (or the "trot out all of the toddlers" variant) would more easily answer the overpopulation argument than getting into the weeds on China's population control policies.

To be fair to Senator Sanders, I think his remarks could be more charitably interpreted as saying that the Mexico City Policy unfairly limits women’s access to birth control (which is important for sustainable population growth), not that abortion itself is a vehicle for fighting overpopulation and climate change. On the other hand, he is on the record supporting directly funding abortion as part of foreign aid and didn't take the opportunity to clarify his remarks.
https://www.washingtonpost.com/politics/2019/09/05/bernie-sanders-abortion-population-control/
https://rewire.news/article/2019/08/21/these-2020-democrats-support-repealing-the-helms-amendments-ban-on-foreign-assistance-funding-for-abortion/