It says that kids are inferior goods because richer parents buy less kids.
However, definition of inferior goods are goods you buy less as you get richer. Does it presume that the price is constant?
If the price is constant irrelevant of the wealth of the parent, and richer parents tend to have less kids, then yes, kids are inferior goods.
However, the price is not constant.
The price for raising kids are way higher for the poor than for the rich. Rich people simply spend more money for their kids.
Well, governments’ market distortion. Raising kids usually require marriage or at least some committed relationship. Alimony and child support, for example, are more expensive for the rich and the poor. Governments make reproduction expensive for the rich.
What happen when the cost is constant? Well before 1850, the rich are more fertile than the poor.