We want to encourage the population to grow in sustainable ways, not only in some families that make having lots of kids a priority.
Now, the /parents/. They do have a choice. The general system outlined in Larry Nivin's science fiction universe where exponentially higher costs were associated with children past the first free one (there were also lotteries to fairly distribute most of the rest of the population sustenance quota, as well as the option of emigrating off of earth to unrestricted colonies).
Mostly, however, is the issue that today there are many who do not /have/ children because the imbalances in today's society make them feel unworthy or simply unable to do so.