If approached by their employers about an incident, nannies should be honest and provide a snapshot of what they did that day, Ms. Panagiotopoulos said.

WSJ: Parents Patrol Nannies Online

Helen Panagiotopoulos, a former nanny and spokeswoman for Domestic Workers United, said such cases of fraud are "an anomaly" and a result of the way the industry is set up.

WSJ: When Nanny Ads Lie

Helen Panagiotopoulos, who spent 19 years working as a Manhattan nanny, said that New York nannies are keenly aware that they may be watched on the job whether via a video camera in the home or by other parents at the playground.

WSJ: Parents Patrol Nannies Online

Sometimes postings about their behavior cross the line into defamation, but nannies have the resources to sue a website, said Ms. Panagiotopoulos, who is now a doctoral candidate in anthropology at CUNY Graduate Center, where her research focuses on the power relationship between domestic workers and their employers.

WSJ: Parents Patrol Nannies Online