The only reason the reproduction rate matters so much in the west is because of the impact of each individual person. If we systemically reduced the CO2 per Capita by half, having as many children wouldn't be a problem. There are many other things you can do that have a massive impact; things that are magnified you have any kids, like eating meat weekly instead of 2x per day, or turning down your home thermostat. On an individual and a population level just those two changes can have enormous effects