Assuming your Nanny knew that your child was on a waitlist for preschool - - I am sure she will fully understand if you need to let her go now since it is beyond your control that he got in a little earlier than you had initially thought.
I highly doubt she will blame you.
That being said, can you let her know right now (vs. the end of next wk.) that your child will be starting preschool the beginning of next month?
Because the more notice that she receives > the better it will be for her to secure a new family to work for.
I would let her know stat.
Do you have any idea why she may be less enthused about working w/your family these past six mos.??
Could it have anything to do w/her pay scale or household duties??
Regardless it might be a good idea to show her how much you appreciate all she has done for your family w/a nice Christmas bonus and/or severance payment.
Wishing you all the best!