Do you have any reason to believe that the nanny is showing them television shows during the day other than the fact that your child knows the names of these characters? If you are assuming that your child has seen these shows because your child is talking about the characters, singing the songs, and is familiar with some of the narratives involved, then you really need to delve a lot deeper before making any assumptions. My kids have never seen a single episode of Dora, but they can sing the theme song, they know who she is and some of her catch phrases, and they know some of the parade for all characters as well. Their peers are interested in this and have books and toys featuring their favorite characters. I agree that the nanny should not be showing the children TV shows, but the fact that your kid has heard of these characters is not enough evidence to condemn them