The reason society might have a compelling interest in supporting childbirth is to support new tax payers (i.e. kids who grow up, work and pay taxes) Those taxes go toward maintaining existing services including Social Security for retirees. If there are too few paying taxes the whole system falls apart. Here's a link to an article on countries experiencing negative population growth. And here's a wikipedia entry on Population Decline - the sector titled, "Economic consequences" is a good historical overview. It also covers what can be expected when a country either doesn't have a replacement rate or has a declining population.

So it's not only a selfish position to think government funded childcare and basic income are good things. They can benefit society. And yes, 'the devil's in the details'. That's why there should be thoughtful experiments to see what gives the greatest benefits to the greatest number of people.