This is the silliest argument for why intelligence would be negatively correlated with number of children. FWIW, the original article very much did not assert that assertion; reproductive success (meaning having any children at all survive the parents) in fact may be positively correlated because the social status of richer/more intelligent parents translates to higher survival rates for their offspring.

Regardless, while the absolute cost of raising children may be higher for high-income families, the share of their total income that represents is much lower. Someone earning minimum wage will struggle to support even one child's basic food, clothing, and shelter; their counterpart making $1M a year could easily afford private school, childcare, and the best possible opportunities for a family of 3-4 children.