My thought process was that going from 1 to 2 was going to follow some sort of logarithmic increase in complexity - but I am now convinced that the change is exponential.
With 1, you can split the work. With 2, the dad will probably be focused on the older kid and the mom on the younger kid. Thus, there is less help to go around.
Maybe at #3 there is some sort of loop-back effect where the older kid can help out with the younger siblings?
Anyone with more experience care to opine?