It's a tricky one because with so many other people in the house you cannot be sure what has happened to the money.
I would perhaps put it down to experience because it is unfair to accuse someone of theft if you have no evidence. By the sounds of it your nanny is not having a great time of it at the moment so I would perhaps tread carefully.
Of course you will have to keep an eye on things and it may be wise not to leave any money lying around. Why don't you ust get into the habit of asking her for the change? That way she can't keep it.
Sorry don't have any real experience with this kind of thing but do appreciate that its a tricky situation.