This being Hacker News, we can talk about it in terms of graphs, right? Let's look at the sociogram when one parent is present.

The transition from zero to one is of course world-changing. But at one child, 100% of the connections in the sociogram connect to a given parent: zero economy of scale.

At two children they have each other, but that is only 1/3 of the children's connections when one parent is present.

At three, we're up to 50% of the connections that are child-to-child. At this point the sociogram is rich enough to become interesting.

Four => 60%. Eight => 77.8%.