I think the dichotomy you are speaking about is wrong. It's not about having or non having kids, it's about how much kids you are going to have. Most couples have indeed at least a child, but a couple is formed by two individuals. Which means that if they have less than one child the population is going to decrease. However when you have multiple children you are going to split up economic means and get additional economic burden. That's the problem. Most whites sets with 1-2 kids and call it a day. That's not enough. But in order to keep social status having a single child is better. We would need something like ancient regime institutions - planned marriage, military jobs for any make except the first, things like that - to counter that trend.