Children and wealth form a U-curve in the USA. Poor people: lots. Rich people: lots. Middle class: few to none.
The middle class is in the unique position of craving upward mobility while being given the majority of the tax pressure and increasingly narrow means of differentiating a child among other children in a global market for an educated class.
They also form the bulwark of conspicuous consumption dollars even if tastes have become more refined over time to not plainly state it as such.
These are all factors which may correlate overall with societal wealth but are nonetheless not necessary to it, nor may it be necessary to abide to these incentives.
I can see a conservative middle class producing many children but mitigating these expenses through greater social cohesion – which could mean more connected networks to take advantage of for labor placement rather than costly degrees to signal to HR departments that can't afford to put faces to names; or maybe having more people to rely on if you need to alleviate the time-pressures of raising a child – as well a different sense of what makes life worth living beyond one's toys or "experiences" which are passive engagements with scenery or momentary exhilarations.