If I don't want children of my own because they're ridiculously expensive, why should I subsidize yours? There's no reason a private employer should pay you more because you want a kid.
Children are not personal property, they are a cultural resource, and it is perfectly acceptable to make people invest in the important parts of their culture. If you don't like it, give back all your state-printed social-agreement-enforcing cash and make your own monopoly money to hoard. You, as a participant in an economy, have social responsibilities. Caring for the next generation is a big one.
Yes, thankfully we have to pay for our own children's education, transportation, food, and healthcare because nobody pays taxes. Truly, this makes the US a wonderful place.