OP, I completely overlooked that this nanny hasn't worked legally before. In that case, she has no past tax returns, and she does need to just figure the "normal" amount of deductions to take and then next year she can adjust things.
So forget about the tax adviser at this point.
There are numerous tax services that can handle the entire thing for you. My employers use HomeWork Solutions. My understanding is that there is a very minimal cost increase to add withholding of Fed/State taxes to the SS/Med withholdings, and as long as nanny does indeed INTEND to pay taxes, I would strongly suggest you withhold everything. That will make life MUCH easier for everyone.
Good luck to you!