“For the record, my neighbor had a nanny that fed the children treif. My sister has a friend that had a nanny that was watching the kids on rosh Hashana while the parents were davening and put on a dvd to keep them occupied. Those 2 stories come to my mind and I am sure there are plenty others.”
Everyone has nanny horror stories. I’m sure there are frum nannies who forget to feed the kids, or leave a huge mess, or snoop, or steal, or put the kids in front of the TV all day while they do their nails, etc etc. There are probably also people who are not Jewish who pretend to be frum, or people from frum families who are secretly off the derech, or whatever.
Bottom line: don’t entrust your children to just anyone. Check references and go through a “trial run” while you or your spouse is at home. There are definitely reasons not to use a non-Jewish nanny, but this is not one of them.