Well, if you have one person working and another raising and educating their children, you get to tax one source of income.

If both partners work and therefore need to hire someone to watch their children, you get to tax three sources of income.

Also, if children are kept separate from their parents as much as possible, they form relationships with educators and state institutions rather than relatives, growing up to naturally and effortlessly perpetuate an immoral system of state control of private affairs.