Let me give you some very important advice her OP and please take it because it will make or break the situation at hand.
Timing is EVERYTHING here!!!
In other words, let her know ASAP. The sooner, the better. The longer you wait, the worse it will be.
Thanksgiving is next month so you need to tell her like tomorrow. I am sure considering the circumstances that are totally unexpected and out of your control, she will not have any issue with it. She may even be happy for you.
However, as each day passes, if you do not tell her, she may be less happy.
So let her know NOW and even offer to train the next nanny for her.
By the way, congrats on the new position and I wish you a huge amount of luck in all of your future endeavors.
It sounds like you have a bright future ahead of you OP!
