Any ideas about how to achieve that? An n-child policy? Forced sterilization? Property size limits? Maybe you'll just be kind and suggest punitive taxes.
^ and improve well-being of children, and improve the sustainability of the planet
"U.S. Population Growth and Family Planning: A Review of the Literature" is still the best collection of potential methods, as far as I know despite it's age.
Make people wealthy enough to live comfortable lives and get educated.
Marginalize the people who get in the way of that goal.