Remember that this is a business arrangement, not necessarily a personal one. Without a contract, you can terminate her services at any time. Most licensed providers use a contract (which also limits their personal vacation time) with specific parameters about ending the contract. For us it was "2 weeks paid notice, unless the provider had a child on the wait list and the spot could be filled immediately".
When we switched our son from in-home to preschool (for very similar reasons), we gave two weeks notice and switched him. This is the "fair" thing to do. You could offer to do a 3/2 split for two weeks, but that would be up to you entirely.
If you can swing it, offer to pay her one week's salary and send your daughter to the preschool that week. It will mean "double tuition" for a week, but it may also leave that other door open if you have more children in mind!