[riffraff]

this is more or less what Hungary does: you get a lot of stuff for making kids, e.g. you get 10M HUF (~$35k) if you make a third child, and 3MHUF (~$9k) to buy a car. This is of course much more significant in Hungary than in the US due to cost of life.

But it does not seem to have (yet) significantly impacted population decline for now.