Become a developed nation and make children a cost vs an economic benefit. You don't even need to be a developed nation, look at Iran if you want an example of how to do it.

A combo of education, women working in the workforce and easily available birth control and birth rates drop below replacement rate naturally, no special laws required.

After learning this, I'm really not worried about overpopulation as an environmental problem. The solutions are relatively easy to achieve compared to something like global warming.