Yes, you can use dependent fsa monies for a nanny. Ask your fsa provider what forms they require.
Mine requires a form completed by me and signed by the nanny certifying what she was paid and in what capacity she works. Sometimes I've submitted the W-4 tax paperwork for documentation but recently all they've wanted is the form. (Which could totally be faked and is far less solid evidence than tax paperwork, so it's irrational, but that's a rant for a different day.)
