A transgender person, or a tranny, who also happens to be a nanny. This is mainly a woman who wants to be a man (sometimes vice versa) who cares for children. Being a nanny is a suitable job for a tranny, because their real gender could be kept a secret if he/she wants it to be. He/she may appear to be a cross-dresser.
Mrs. Smith: "I've become a little suspicious of my kids' nanny. I thought she was just a cross-dresser, but now I'm having doubts."