Hi, I do agree with the others and think you should speak to your nanny, but perhaps in a more informal way if it's possible.
My own dd came home from nursery one day and said J, the pre-school room leader (male) had slapped her. I know all the staff at nursery very well (my older son had already been in his class etc) and so couldn't really believe it. I did bring it straight up though and said "dd said you slapped her yesterday, what is she talking about" (I sort of said it in a nice way !!), J said that dd had been told off for a small misdemeanour, but had definitely not been slapped and I totally believed him.
In the following weeks it became obvious that dd was misunderstanding the word "slap", and in fact mixing it up with "told off". She said things like "mummy, don't slap me, I will tidy up in a minute", and "I like granny, she never slaps me". She mixed up the phrases a lot, and often when talking to other people - it didn't portay me in a very good way !!
I do find it hard to believe that a nanny who has been with your for a reasonable length of time, and who gets on with previous employers and is generally reliable would be stupid enough to slap a child across their face, if she really has, then I would hope that she had a really good explanation of the situation. I would want to know what had happened, but would keep an overall open mind.
Helen