Why are you looking into providing retirement plans for your nanny, if you're unhappy with her and looking to fire her without severance?
http://www.dcurbanmom.com/nanny-forum/posts/list/349398.page#3291562
Similarly, you've asked about providing health insurance:
http://www.dcurbanmom.com/nanny-forum/posts/list/349355.page#3291378