For example, Wauchope's sitters are not allowed to watch TV but must engage kids in educational play. New York parents are especially nervous about the E.R.B., also known as the S.A.T.'s for three-year-olds, which is used by private nursery schools as a factor in admitting or rejecting children. Parents not only increasingly ask for babysitters conversant with a foreign language—French, Mandarin Chinese, and Spanish are the most popular—but also insist on fluency.

"It's not just sitting on a couch and watching a TV show until the kid is asleep," says Stacy Jones, an Upper East Side client of Sensible Sitters who recalls her own days of TV watching when she was a teenage babysitter in Ohio. "The caregiver is stepping in and really managing while the parents are at work."

Professional babysitters are also just that: more like professionals. Many of Wauchope's sitters are required to file a status report after every shift. They tell Jones, for example, if their charges went to the park, whom they played with, and what they drew in art class. Jones says she's even asked for a full accounting of the number of diapers her younger son uses up, so that she can tell if the boy's digestive process is on track.

One of Wauchope's most sought-after sitters—a former Manhattan preschool teacher who asked that her name not be used—says parents often ask her to help their tiny tots prepare for the E.R.B. To improve "fine motor skills," one of the tested areas, she gives her charges toy scissors and nudges them to snip away at a lump of Play-Doh. She also arranges an ordered rainbow of M&Ms and encourages them to predict "what comes next," so that they learn to recognize patterns, another tested area.

Parents pay Sensible Sitters directly, starting at $18 an hour for one child. After taking a small cut, the agency pays its sitters. A sitter overseeing several children at once can make $20 or more an hour, plus tips.

Last summer, Sensible Sitters expanded into Long Island with a Hampton Bays branch, where a dozen or so sitters remained on call for vacationing families. Wauchope has grown the business largely through word of mouth, a lesson she gleaned from watching her entrepreneurial parents run their custom-furniture business. She attends parents' group events and creates goodwill in New York, for example, by donating babysitting packages to fundraising auctions at various private schools.

"New York City is very small. One of our Upper East Side clients referred us to her interior decorator down on Chambers Street, whose child goes to St. Ann's in Brooklyn Heights, and St. Ann's has a high concentration of people in the art world. So we picked up a lot of St. Ann's clients," Wauchope says. "In Tribeca, there's a huge baby boom right now. It's all about referrals and networking. We don't really need to advertise."