My boys were 7 & 8 the 1st time we brought them to WDW so the face character differences wasn't such a big issue. I have had questions similar to this in the past when it comes to Santa, Easter Bunny, etc. My advice is just get creative. We always made elaborate stories to explain nuances they noticed. With the princesses you might whisper in your daughter's ear: "Well, what happened was that Cinderella had some Princess duties in (name another park) but she really didn't want you or all the other little girls to be sad if she wasn't here to see you. So she sent her royal bff to meet with you and play pretend. We don't want to hurt the other little girls' feelings so we have to keep it a secret. Can you help? Playing pretend is fun." Or, if that would be too complicated just tell her you heard she's trying out a new Princess make-up that made her look a little different today. :king:
Personally, I loved telling my boys little things so they felt like they were "in" on some secret grown-up detail and they were being extra sweet to other kids by playing along and not ruining any magic for them. I think that worked best for us when they get a little older like around 6-8 years old. It also taught them the importance and joy of giving the magic to others and creating what we all hold so dear. Now that they're 11 & 13 we can sit in restaurants like Crystal Palace and they actively notice, enjoy, and comment about little babies or small children when they meet characters. The expressions of discovery, wonder, and joy are noticed and mean something to them even tho they have no connection with the other children other than being in the same place at that given moment. It's really sweet!