OP, I think you should speak to someone. I like the idea of speaking to a teacher, if any of them are around to observe in the morning. That way, as Expatgal says, the parents are less likely to get defensive.
I was just reading a story on Yahoo yesterday about a family dog that started reacting negatively to the nanny. So the parents hid a recording device in the couch cushions and caught the nanny on tape, being very negative, borderline abusive, with their son. They confronted her and she was fired.