Would this be as part of a larger employment agreement/contract? Or are you letting your nanny go and spelling out details? Not clear from your question and I think it makes a difference...
We have had 2 nannies, one for 3 years, one for 2 years. When we hired we spelled out all kinds of details related to employment, benefits, compensation, etc. We also spelled out our request for 1 months' notice if she left, and agreed to provide at least one months' notice if/when we were ready to let her go (unless for cause). In the unlikely event that we had given her less than 1 months' notice we agreed to pay the difference. There are lots and lots of pretty straightforward agreements like this online.