It would be a part of the agreement up front that if you were not already a parent you would only be allowed to have one kid. Two parents and one kid coming through the gates. If you had a kid but remarry the one kid rule would still apply. Since two people allowing one person to come through their doors leads to a decline in population overtime, new recruits would apply for residency to fill in the loss.
The Area would be an agent of gentle depopulation and death powered by consent. If you have more than one kid without exempting circumstances you would lose citizenship. Certain exceptions may apply, including but not limited to twins, but they would be rare. There would be state paid abortions if the pregnancy is caught early enough, but not after a certain point. I am going with Aristotle and some Indian guru who said 40 days. I have not been around a pregnant woman and been close enough to them or aware enough to feel when ‘personhood’ is established in the fetus myself so as it stands i must trust this. This would be a harder experience of life than we modern folks know.
A parenting class would be required for couples who are pregnant. Having a lower population than today’s is better for humans as a species ( right now we appear to do what bacteria does in the Petri dish. It eats the food the scientist/experimenter gives it, booms in population and then collapses due to unsustainable use of resources to support itself.) and it would be better for the planet.
Conscious population control, something not enforced but decisions made by the individual to limit or have no off spring is ideal. Part of the sign-up agreement would be this discussed directly above. If you want to have kids, okay, but not in this place. It would also be advised that people note their own health and family line and decide as conscious beings whether they should pass on their bodies and their genes story or if someone else may have the honors. It would be asked that people think of this. There wouldn’t be any crazy mandatory genetic testing with a pass or fail criteria, people would be asked to review their own and families health including body, mind and spirit.
Just asking for a moment of reflection, of thinking of the best vehicles for the future living. If this society is to last into the future, people must think of the collective story of the participants and the human species and not only their own.