Hi T.,
As the others have said, I would let her know as soon as you know. As someone who has not only used in-home care in the past, as well as being an in-home provider now ... I have always given my babysitters notice as soon as I knew that things would be changing, I thinnk it should be at least two weeks, if not more. For instance, I had someone watching my children when I was working outside the home and it ended up that I would be quitting my job, I gave her the same 2 week notice that I gave my boss. Well, things happened and I didn't end up needing her the last week, but I still paid her for that week because she was looking at having that income during the time, so I thought it was only fair to her :)
As a childcare provider now, I would like to have as much notice as possible, it's very hard to fill a spot sometimes and it's nice to know ahead of time when you'll have that opening, so you can get to work filling it as soon as possible. There are many childcare providers out that that depend on that income and have a very hard time going without it for any amount of time, much less when it comes unexpectedly.
Good luck with everything :)