It gets problematic, because then you'd basically have a system where the government decides who gets to have kids.
A better solution is probably a raised standard of living and more education.
edit: By raised standard of living I'm not talking about AC and other energy-intensive luxuries. I mean not having to have 7 kids because many of them will probably die.
Its an interesting question to ponder if the reduced birth rate by increasing quality of life, education levels and general health will offset the raised material resource consumption by that move.
In the end I think the individuals themselves will be the ones making the decision to have fewer kids. I don't have a source for this, but I seem to remember that immigrants to more developed nations have fewer children after assimilating than people in their native, less developed countries.
Several principles suggest not. The Jevons Paradox, White's Law (after Leslie White), and the Darwin-Lotka Power Law.
These seem to reflect strong underlying tendencies of complex evolving systems. In particular that higher levels of organisation and complexity very powerfully tied to greater rates of energy and resource use, overall if not individually.
If humans manage to defeat this tendency it would be a singular exception.
A better solution is probably a raised standard of living and more education.
edit: By raised standard of living I'm not talking about AC and other energy-intensive luxuries. I mean not having to have 7 kids because many of them will probably die.