a political class has no moral right to impose on its population decisions such as how many children should they have
indirectly however, since there are consequences from the externalities of excessive population, it makes sense that people should pay for them as close to the problem as possible, as they do with basic education and hospital care through taxation
indeed people already make decisions on how many children they should have based on the fact that children are no longer cheap labour and are in fact a massive liability in economic terms at least for the first couple of decades of their life, as children are not allowed to work and parent have a duty to care and to provide for State-mandated education among other things in most if not all of the industrialised world
these liabilities already constitute a large tax on children, perhaps too large in many cases as people have decided to have less than 2 per couple, leading to an aging population
the framing of this question implies that the political class has the right to decide how much population is too little or too much, rather than the very effects of population numbers on the people - removing agency from the people and assigning it to the State through the political class
at most, the political class could perhaps pose the question to the electorate, perhaps as part of an electoral programme, to take measures that would lead to different incentives to have more or less children; but I don't think this is even an issue when, as soon as the externalities of children are exposed to parents in terms of long-term liabilities from having children, the problem is that people may have too few rather than too many children