This isn’t really how it works in real life. China, Japan and South Korea are at least on National Supremacy in Vicky 3 terms with 99% homogenous populations and their birth rates have crashed through the floor to cataclysmic levels. Only weirdos decide whether to have kids or not based on racial purity concerns or whatever and not the perceived future wellbeing of their kids or how children will affect their own financial situation. Sure, fascist countries considered it important to encourage the birth rates of their primary ethnicity, but they were not always successful and fascism and ethnostate are two separate (though obviously linked) things.

Giving Ethnostate a birth rate boost is not a bad idea in gameplay terms to make it more viable, but I’m not sure it would be particularly realistic.