Full-time day care for two pre-school children in the U.S. can cost $35,000 a year or more. A live-in nanny can cost even more. High prices like those are why thousands of parents turn to the State Department’s au pair program, where young foreigners provide low-cost live-in care in what is called a “cultural exchange.” But not everyone thinks it’s a fair trade, as Veronica Balderas Iglesias reports in Part 2 of her series, “The Perils of Au Pairs.”