>My take on it is: you have to make your country/society a place where people will want to have children and feel/know that their children's lives will be good ones.

There's only one developed country with a birthrate above replacement and that's Israel, which is hardly a paradise. Largely due to Ultra-Orthdox Jews, who believe they have a religious duty to have children. Empirically religion is the only thing capable of making people in rich countries want many children, and religiousness is partially heritable so eventually the problem will solve itself as the secular-inclined genes are bred out of existence.