This won't work. You have no instrument on actually limiting child birth and no one ever will have it. While some nations might listen to that advice, others simply won't. Then what?

We already have a method for reducing birth rates that has worked 100% of the time - end extreme poverty, send girls to school and provide basic healthcare.

https://ourworldindata.org/fertility-rate

This method somewhat works. But the person above really thinks asking people politely would achieve anything.