You could be my co-nanny share mum. well, almost - our child who is 2 has started scratching and has scratched her nanny buddy twice in the past couple of weeks. She's been sick, and very off-colour, but it doesn't make anyone feel any better about it! I feel guilty, the other mum is understandably tetchy, and the nanny, who is brilliant, I think feels like her professional skills are being challenged!
If it is happening A LOT then you need to talk to everyone about it - we all had a chat about it when it had happened once just to clear the air and to make sure that it is addressed identically by everyone (the nanny and us when it happens, for consistency's sake). Then you can have a strategy. One of the frustrating things from our perspective is that 1. we really are doing everything you are supposed to do and 2. it's very out of character for DD, whom you would never guess is a scratcher. She is not boisterous or particularly physical! So we are at a loss a bit about what to do, other than to try to be as consistent and tough as possible. I've also called the other mum so we have talked it through. I'd be so sad if she wanted to terminate because of it, everything else just works beautifully.
Our nanny also pointed out quite sensibly that this is one of the major challenges of a share. If this were siblings, with one scratching the other, you would be cross but probably not feel the same way about it!
If I were in your shoes, I'd want to know that the other mum felt bad about it and was doing things to try to overcome it.
As MrsFlux says it might just be long nails, as well!