Nannies in many parts of the country work for several years, often until the need for a nanny is replaced by the need for a short after-school commitment, until the parents come home. Most initial contracts are for a year. The American nanny is not a third parent, nor a parent replacement. Incidentally I feel that hiring a babysitter should entail the same background investigation as a nanny whether the sitter is a friend of a friend, a teenage neighbor or a classified respondent. There are some questions about the social behavior of nannies, like drinking under age, driving while intoxicated etc.
All states require workers compensation.
Families that do not provide this can be charged with negligence if an accident occurs on their property. Even illegal aliens have successfully sued parent employers for damages through workers compensation. Shortly after I started my book, several books were published about hiring nannies. Three murders of innocent babies on the east coast during my ten year tenure are the rare horror stories but I met several nannies through play dates that openly admitted they had falsified documents to get their jobs. One lovely nanny who worked full-time for a prominent New York family was leaving after four years.