Here's the quick answer and it is not good news:
Nannies, au pairs, and mother's helpers who are accompanying or following to join an employer who seeks admission to, or who is already in, the United States in B, E, F, H, I, J, L, M, O & P, Q, or R nonimmigrant status may be eligible for B-1 visa classification provided:
The employee has been employed outside the United States by the employer for at least one year prior to the date of the employer's admission to the United States, or if the employer-employee relationship existed immediately prior to the time of application, the employer can demonstrate that he has regularly employed (either year-round or seasonally) domestic help over a period of years preceding the time of application;
The employee has at least one year's experience as a personal or domestic servant as attested to by statements from previous employers;
The employee has a residence abroad which he or she has no intention of abandoning;
The employer and the employee have signed an employment contract which contains statements that the employer is guaranteed the minimum or prevailing wages, whichever is greater (Further information is available from the Department of Labor?s website at www.dol.gov), and free room and board, and the employer will be the only provider of employment to the employee.
This is all from this page: www.usembassy.org.uk/cons_new/visa/niv/aupair.html