You'll need to prove that you have had the nanny working for your family for the last year. Receipts are ideal.
You'll also need to sign an agreement with the nanny saying that she'll be paid at least the minimum wage in whatever state you'll be visiting, and she'll only have to work 40 hours per week and will have breaks and such. Whether or not you actually follow such an agreement is between you and your nanny, but you need a signed document to that effect in any case.
The nanny will need a passport, of course.
You'll also have to sign something saying you'll be responsible for any and all expenses incurred by the nanny while in the US.
With all of that, you can request an interview for the nanny at the embassy. At that point it could go either way. Depends on the mood of the interviewer, current political climate, how well the nanny interviews, etc. Basically there's no guarantee, but as long as you can get all the paperwork in order there's no reason to not give it a try.
Suggest you call the embassy for any clarification.