Incentives to have children will never, ever get you to your desired fertility rate. The problem is, if you take taxes and then give it to people that have kids, you're subsidizing having kids at the expense of those that don't. That means you need a large population of people not having kids to afford it, if everyone takes advantage of it then you'll wind up just taking peoples money and giving it back to them, which puts them right back where they were before your program.