In countries with solidary pension systems you can surely bind retirement benefits to number of children.
The issue being, they try to avoid paying more than the bare minimum anyway and if you pay less, that means dead old homeless people on the street. Would be a great advertisement to having children, thoug.
In my coutry, most people under 45 (who could actually bear children) doubt they will see any retirement ever. Of course that affects their desire for procreation as well.
Society would rather take their chances with Gen Y and Millenials than open that pandoras box.