I mean, surely it's fundamentally understood that we cannot have infinite growth with finite resources. We're already causing mass extinctions.
I sound like a hippie, all I really expect from humanity is a little compassion for the people who we bore. It's an infinitely dull planet we leave behind if we continue this trend, and one that's increasingly hostile to our descendents.
That compassion should extend to not interfering with people's right to reproduce. "We're saving you by not allowing you to have children" sounds incredibly hostile.
Similar to how we tax things which we want to curtail use of, some countries might not tax the same thing and it causes an imbalance.
I'm not a politician, I'm an engineer and thus lack a lot of foresight and compassion that might be needed to understand this properly, but if I were responsible I'd probably approach it like this:
Step one: Identify the problem on a global scale, broach the idea with partner countries to enter an introductory treaty to at least acknowledge that we need to slow/reverse human population growth. Make objectives that ensure that our economies do not rely on population growth.
Step two: use the treaty to gain leverage over non-parner countries. Same rules, just acknowledge the problem and set some objectives to ensure that economies aren't tied to growth.
Step three, amend the treaties to slow the increase of the human race, ensure that partner countries bestow penalties on households with more than 2 children. (Additionally: Ensure that single parent households are only having one child, this could cause turmoil otherwise)
Step four, push for the reduction (or removal) of any benefits related to a second child birth, remove any/all income benefits for a second child, increase the benefits for a first child (must be a first child for both parents)
Step five, attempt to promote having a second child in media as a "very selfish" thing to do.
Step six, apply heavy taxes on dwellings more suited to 4 people families, attempt to reduce creation of such dwellings though tax incentives.
Step seven, finally, if everything fails, tax the families of those with more than a single child born after the date where this law is ratified.
If you really want to help, all you have to do is lobby to support economic development of the places that are the feedstock of western country population sustainment.