C..
There seem to be a significant amount of rules with hiring an au pair.
http://en.wikipedia.org/wiki/Au_pair
But if you can keep all of that straight and you are OK hiring someone from a foreign country for a contracted amount of time, then I think it would be a good cultural experience for your kid(s).
Personally - I didn't want anyone to "join my family" which is the expectation of an au pair. Plus, they have to have some educational experience while in America and that would not have worked for my job schedule when my daughter was that age. I needed someone available during traditional working hours.