I don't think it'd be especially hard. Ban contraception, economic coercion, mandatory family planning at a certain age and you're probably already there. For the economic coercion part there are endless forms it could take. An obvious one would be a 'national sustainability' tax assessed on the number of children one has (adjusted for minimum age) - 40% for 0, 20% for 1, 0% for 2, - 10% for 3, and so on.

Another way would be to fine companies and local jurisdictions if their average number of children per employee/resident is below 2.1 or whatever, similar in spirit to Japan's 'metabo laws.' You could also require companies prioritize employment, beyond a minimal standard, based on the number of children per employee, and so on.