[brc]

A combination of rarity, utility and sunk costs.

The article seems to be wandering down the tangled nest of errors called the labor theory of value, or one of its offshoots. This bedevilled people for a long time who could not explain why a diamond was worth so much when it cost little effort to make and is objectively useless.

Modern children are diamonds. They are not that difficult to make, mostly useless but give great utility to the parent. Modern children are also rare, thanks to birth control. Most affluent societies are below replacement rate, while 1800s societies had kids everywhere. The most spoiled and indulged children are the ones who don't have any siblings.

So... - kids give their parents pride and joy (like a diamond) - kids are relatively rare compared to the prior period - the upbringing of a child is 20 year project for parents now, so the loss of effort as they get older increases the pain of loss

Most of these points are in reverse to the prior period, when you'd have 5 or 6 children in the hope of getting 2 or 3 to adulthood, which was onset much earlier. You had more, invested in them less, and everyone else had more as well.