Depending on who her favorite characters are:
Chef Mickey's at the Contemporary for breakfast or dinner has the traditional characters(Mickey, Minnie, Pluto....) This is also easy to get to on the monorail or walking from MK.
1900 Park Fare at the Grand Floridian has a Marry Poppins Breakfast and a Cinderella/Prince Charming dinner. This is also easy to get to from MK on the monorail.
Crystal Palace at MK for breakfast, lunch or dinner has Winnie the Pooh and Friends.
(all three of the above restaurants are buffet. They are all yummy too. And each has their own little "celebration")
CRT: I've never done breakfast here, just lunch. They really make the little girls feel like princesses with a few extra goodies including pictures with Cinderella (all inclusive)! It is a set menu though, so check it out before making an ADR.
If your daughter is really into the princesses and your budget can manage it, then I would suggest doing the BBB followed by lunch at CRT. This would def make for a very special day!
Also one of my DD5 favorite breakfast is Hollywood and Vine at DHS. They have the Little Einstiens and Jojo's Circus. The kids get the opportunity to do some fun little dances with the characters. (I know you mentioned MK, but I thought I'd throw this in FYI)
Happy Birthday to
your Princess!