I always think the point of cameras is to prevent anything bad, not gloat in catching the nanny abusing your child. Stating outright you have cameras will turn away any nanny who might be abusive or neglectful (as well as great nannies who just don't feel comfortable being watched by someone they might not even know), so I think it's way more useful, as well as decent, to let nannies know you have cameras straight away.

OP - it does not record audio, and, I did tell her and she didn't seem to care. I barely look at it anyway.

TELL HER ASAP. It is the right thing to do...