Do you need to address this? Yes, definitely.
Do you need to sack her? No, absolutely not.
As has been mentioned on here, there are many reasons, some more likely than others, that mean she may be wholly innocent and has taken the money by mistake, may have thought it was an advance payment for next week, or could even have been a bonus.
Whether or not this is the case, I think anybody deserves the right to be heard before something as life changing as being sacked. There are so many threads on Mumsnet about people who are struggling to make ends meet, please don't send another family down that path needlessly.
And I really struggle with comments of some members on this thread who seem to be painting your cleaner as a criminal mastermind for taking a pile of notes left out on the table where she normally collects her wage. If she was out to fleece you, this is not the smartest way to go about it. I reckon it will all be sorted out after an honest chat, cup of tea and packet of malted milks.