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> No wealthy family is worrying about that.

They are, but in a different way. They worry if they can still afford to pay the mortgage while also paying for a second daycare/preschool tuition and then later a second private school tuition.

> The cost of raising a kid might go up with income, but probably not at the same slope as income, which would mean they get cheaper in relative terms.

FAFSA allows colleges to perform perfect price discrimination, so the cost of raising a kid through college absolutely goes up with the same slope as income. When colleges know you have significant retirement savings and a high income, your kids will get zero financial aid.

In my observation kids with older siblings tend to develop faster. As in they walk at an earlier age. I guess they get fed up with watching the older kids running around and being left behind. Older kids also often act as deputy parents. So I don't think kids from large families surfer from attention deficit. I am a single dad and often struggle to keep my child attention right through the day.

> That’s hard for me to wrap my head around.

You seem to be trying to model social phenomena as linear functions. That's going to fail on almost every time.

The GP is also claiming monotonic behavior. Please understand it as constrained into a "reasonable" interval that he didn't disclaim. Otherwise it certainly won't hold either.