If anything our world is overpopulated. It seems to me we'd be remarkably better off with fewer kids. I reject the notion that systems optimizing for raising kids would put society in a superior place. I'm not against anyone having kids, but I reject the notion that it's a benefit for society that we encourage people to bear and raise children. People will bear and raise children regardless and I think reducing incentives there would ultimately have better societal outcomes. This is reflected all over the world with developed countries having fewer kids per family. Correlation != causation, but it's certainly evidence.
I would guess that the optimal state would be at or just below replacement fertility rates (because immigration will bump your population growth up a bit).
People are going to have kids regardless. So, the question we're really asking is, should we structure things so that people have time to raise those kids that they are already going to have. I haven't reviewed the literature, but I'd guess kids who have their parents present in their life have better outcomes.
