This is a tale of two families.

Some friends of ours have three children, ages 1 to 8. The husband works, the wife stays home with the kids. They're making ends meet, but it's tough. And the wife is starting to go a little stir-crazy from being at home.

So she decided to go back to work. Or try to.  The problem is, the entry-level jobs she can get after nearly a decade out of the work force pay little more than $10 an hour — pretax.

They don't have any relatives in town to help with the child care, so if she wants to work she has to hire someone. And with three kids, the costs add up rapidly. Child care for infants is hideously expensive: $1,000 a month or more. Full-time care for older children isn't much cheaper: $800 or so. After-school care is the most affordable: About $200 per month per child.   That works out to $2,000 a month just in child-care costs. They'd need to earn an extra $11.50 an hour after taxes just to cover that bill.

Even the cheapest option — an inexpensive nanny — would cost at least $10 an hour. Assuming they could find one.

My wife recently went back to work, so we're in the same boat. But our circumstances are different. My salary is high enough that we don't depend on her income to pay the bills, and my schedule is flexible enough that we were able to put her through a one-year college program without having to pay for child care. Upon graduation, she found a job paying $15 an hour. And we have only two children, the oldest of whom is entering first grade, so our costs are lower.

Both women can expect the low pay to be temporary — their salary should climb fairly quickly in the first couple of years, eventually alleviating the financial crunch. But our friends cannot afford the low pay in the mean time.   This leads to an ironic and unwanted result: the people who really need the money from a second job can't afford to get one until the kids are all in school and child care much cheaper.

The side effects are large. For one, you have the psychological effects on the wife of being "trapped" in the home and the family pressures that stem from financial difficulties. Economically, the poorer family falls further behind the economic curve: they spend several more years on one income, which delays the growth of their earning potential, which translates into fewer years of maximum earnings over the course of a lifetime. The poor get (relatively) poorer.

The bottom line: the lack of affordable child care helps keep families poor and serves to widen the wealth gap.