In our school we have a "no characters" policy. What this means is that for areas over which the school has control we do not have licensed characters. For us, it means there are no character toys, books, decorations, or other items. No Dora, Pooh, Elmo, Franklin, Fancy Nancy, or anyone else. This policy does not extend to children and families; children can wear character clothing and parents can bring in snacks sporting licensed characters.
There are many reasons for the "no characters" policy. One of the most important, from our view, is that a character limits the parameters of play. It's not as imaginative to act out a tv show or video as it is to create the setting and rules cooperatively. It also prevents children from practicing the social give and take that's required from purely imaginative play. Those are critical socials skills that lead to success in the elementary grades and beyond.
Here I'm going to quote something from Buy, Buy Baby: How Consumer Culture Manipulates Parents and Harms Young Minds by Susan Gregory Thomas (a book I wish I'd had before I had kids). Backed up by research cited elsewhere in the book, she says, "the main thing that infants and toddlers learn from such characters, whether on television, juice boxes, or bed sheets, is the ability to recognize them -- which should not be confused with actually learning anything 'educational.' "(p. 143) What she's saying (and that's backed up by research) is that kids don't learn anything from these characters other than to recognize them and later want them because they're familiar. That's right, they don't learn new behaviorial skills or even much in the way of academics. What they do learn children are unable to apply outside of the story on tv without significant adult interaction. So, when these characters are available in the classroom it amounts to free advertising and a tacit approval of commercialism over education. I don't know about you, but if I'm going to advertise something I want to get paid!
Children who spend time watching shows and playing with character-limited toys are not only not learning the important social lessons that they need to learn, but they are not spending as much time on more developmentally appropriate and "educational" activities. Characters are part of our culture that isn't really helping our kids get ready for school. In the case of things like Hannah Montana and High School Musical in preschool, they send the message to young children that they are allowed to behave like teenagers. I don't have anything against those pieces of entertainment and I personally think they have some good qualities, but I don't want a 4-year-old child to get the impression that the sort of behavior displayed by those characters is appropriate for him or her now.
Now don't take me for a character hater. I've watched my share of Little Einsteins, Dora, and Sesame Street. I've been known to say, "seatbelts buckled, everyone?" in my best Ms. Frizzle impersonation. I've got a fairy/princess lover for a daughter and a LEGO Star Wars obsessed son. But I don't do characters at school. And I'm cool with that.