As an echo to all said above, the themes of princess movies are not for preschoolers, and the dolls are less appealing without the story. We so far at 4 have skipped the stories, the movies and the dresses. We have 2 dolls, both gifts, and the chief appeal is the haircolor that matches our own.
So instead we have stories that are age appropriate, and make sure to dress up and enact those. (Beatrix Potter and AA Milne mostly) So we have a north pole at our house, and "expotition" boots. We have seen Disney pooh, it is one owned by friends and we specifically watch it there. IT has become a nice special connection with those friends.
We have been to princess birthday parties, but chose to have a ballet one ourselves. When princesses come up, we play castle, involving tent-castles, kings, queens and barnyard animals. It is more elaborate and more interesting. And it enables us to play princess with others who desire it. I have found there is very little content to "princess play" once everyone is dressed and providing some is welcome. So we both supplant the stories and extend the stories.
Later will we watch the movies? Probably, but only when old enough to read non-Disney versions of the story first, and then ask how well the movie told the story. The scary factor is huge on screen, but we do read some scarier things, because we can pause and engage it - Is that scary? What part? It's never what I think it will be anyway. Which is enlightening. Reading the stories can provide enough connection for conversations with other children who will talk about the stories.
We did turn down a preschool because the teacher called our playdough mermaid "Ariel" right off the bat (it was actually the mermaid that "lives in the fountain" downtown thank you, if only she'd asked!) and because another teacher commented that it was "okay if they didn't know the stories fro the beginning, because by the end of the year they all know them, and they can act out all the parts." We didn't go there.