All of them are tied to money, really. If you are rich, then you can afford nannies (or a stay at home parent, this is nowadays a luxury too), housing, and if you are in a district with high property taxes, your public school is well funded. College is also easier if you don't rely on scholarships or on your children to work so that they can support you. Health care also suddenly becomes affordable with money (although in the US for big things it's still inaccessible).

In general, if you get N children, then you either favor one child, or each child gets 1/N of the inheritance. Same goes for money available for sending your kid to college. Meaning, as a rule of thumb, the more children you get, the poorer those children will be from that point of view. Like a single can live well from €2k euro per month in most of Germany, but for a family with 3 children the same sum is very little.

And of course there is the entire dateability aspect, too. People with higher income are seen as more attractive than those with lower income.