Yaya Unexpected Time Off

Hello! I'm in a bind! My yaya unexpectedly asked me today Friday that she'll be taking the time off on Sunday, Monday, and Tuesday because she'll be going home to attend her niece's birthday. I feel this is so unfair to me because I have work on Monday and Tuesday and no one else can look after my toddler. She is a very good nanny to my toddler, and it pains me to say "no". I don't want her to quit because of this. However, my job would be compromised if I say "yes". Plus it was on a short notice! What should I do? How should I approach this? Thanks for your help.