There might be little girls out there who would balk at the mention of dressing up like a Disney princess, but they're few and far between. You and your daughter can dress exactly alike, or you can watch a few classic cartoons for inspiration on coordinating costumes.
For example, your daughter could be Snow White while you rock the Evil Queen's trademark make-up and crown. If both of you wish to dress as Disney heroines, consider going as Princess Jasmine and Belle or as Rapunzel and Tiana. Naturally, any little girl would jump at the chance to sport Cinderella's ball gown in tandem with her fairy godmother. If you're feeling really charitable, you could dress up as one of Cinderella's singing mice, but that might be going above and beyond the call of duty. Even mothers have limits!