Once the retirement system is socialized or provided by insurance the role of kids for future retirement support immediately goes out of the window and so is the ROI. You privately spend 1 million dollars each to raise them, but society gets to reap the rewards collectively. It's a losing proposition and only a non-smart person would commit to such agreement.
If countries want to raise their birthrates they should immediately liquidate all their pension funds and insurance funds (both public and private) and transfer the stock/bonds owned by such entities to the holders of such rights.
Also countries should stop policing how people raise their kids. Many parents might want a slightly more ignorant kid in the realm of philosophy/English/math... but prioritizing values of family and most importantly the only value that matters to them: taking care of parents as they enter old age , when I say taking care I mean both financially and socially.