First let me remind that normal goods are any good for which demand increases when income increases, and falls when income decreases given price remains constant. Or put another way, any good with a positive income elasticity of demand is a normal good.
Then let us see what Mr. Wolfers tells us in his blog. First look at the international-wide data(first graph).The discrete points (each of them represent a country) constitutes an almost perfect negative slop line, which indicates that those who born in rich countries (have higher income) tend to have fewer kids than those born in poor countries. Second graph focuses on different countries in detail, which shows that when a country becomes richer, the less kids their people will have. The third graph uses a cross-section data, comparing rich and poor American. It also shows that the rich have fewer kids than the poor. Based on such many data and empirical evidence, Mr. Wolfers’s conclusion: Kids aren’t normal.