I think the cost of children in parental time and money goes up more slowly than linearly with the number of children. Most people in the U.S. buy the education of their children through the purchase of a home in a good enough area. Once they have done so, they don't bear most of the cost of pre-college education (although I am well aware of transportation and other activity fees). We paid our live-in babysitter more when we had three children but did not triple her pay. It is known that having children reduces mothers' earnings, but I doubt that having three children reduces earnings by three times as much as having one. The direct cost of music, sports, and other lessons is proportional to the number of children, but the time cost of transporting them to the same activity is not. Since our youngest turned five or so, our children have been able to entertain each other for hours at a time without needing our attention. We don't need to schedule play dates.
The main benefit of having children -- that they get to enjoy a life -- is proportional to the number of children. If the cost of children is less than proportional to the number, that is an argument for having more of them.
