Very wealthy people in developed nations will often have large families - see Musk, Trump, Zuckerberg. When money is not a concern, many people will choose to have large families.
I've noticed the possibility of a bimodal distribution curve (i.e. like a pair of boobs, Tetons, etc), where lower income has more kids, middle income has fewer kids, and wealthy have more kids (y axis = number of kids, x axis is income).
From my anecdotal observations, I've seen people at the low end of income have a lot of kids because they're largely low-investment parents. Middle and high income are high-investment parents. Middle can't do proper high-investment (have a parent stay at home, live in a nice area so kids can go to good schools, etc). Wealthy can do all those things.
In absolute numbers, there are less high-investment capable wealthy than low-investment poor so the absolute number of children would skew towards the poor.
The "hollowing out of the middle class" could be due to a lot of factors. One significant one might be that middle tier skills yielding middle class incomes are becoming obviated due to technology.
Maybe also a larger number of middle tier ability people.
Also, DOGE savaging the federal government and contractors didn't help, because I think that's where a lot of middle tier people were or sought to be. That job and those sectors are partially jobs programs for that tier. I'm a little skeptical of UBI and more in favor of jobs programs because I think people need activity and missions. Indolence has its own dysfunctions. A "bloated civil service sector" is the bogeyman of a lot of center-right economics, but... with advancing technology, what are these people going to do? You force a middle tier type into retail customer service and he's not going to reproduce. As far as trades go, that's skilled physical labor and again, it takes a particular sort to want to do skilled physical labor, and the office clerk who is less physically oriented and more intellectually oriented is not going to want to do that, and if he has to, he again is not going to reproduce.
We need to see some real non-agenda-driven data on the issue, but these are some possibilities.