Say what you will about Hungary but I think it came up with some great incentives for future parents: income tax cuts. If you have 4 kids you don't pay income tax for life.
because money by itself isn't the issue. otherwise poor people would not have kids. the real issue is that having children conflicts with having a career and other interests.
The childless should pay enough to ensure there are younger people to do the work of supporting the childless when retired.
Using foreigners to fill a demographic shortfall is unsustainable/shortsighted.
That said, I'm in New Zealand and too many young adults emigrate because (A) our economy sucks and (B) richer economies like to employ NZ citizens (NZers love international travel/work).
https://www.theglobaleconomy.com/Hungary/Fertility_rate/