I don't know which Princesses are her favorites, but I would highly recommend spending some time in World Showcase at Epcot in the afternoon. You can see Snow White, Mulan, Belle (blue dress), Jasmine (sometimes with Aladdin), and Aurora/Sleeping Beauty in the different countries and the afternoon. The lines for the princesses aren't usually that bad there. Be aware that they do come and go. You will want to get the times guide, and if it doesn't have the specific times they are out (i.e. if it says 11-5, in stead of something like 11, 12:30, 1:30, 2.30 etc), you might want to check with guest services to find the specific times so that you don't miss the ones you really want to see as you are walking through the countries.
If your DD wants to see Cinderella (and you can't get a meal booked), then you will have to see her at the MK (on Main Street currently). They have fast passes for that meet and greet now to make it easier. I haven't been there since it became fast pass. Every time we have done the Princess meet and greet at MK it has been Cinderella, Belle (yellow dress), and Aurora/Sleeping Beauty. You will also be able to find Tiana and Rapunzel in other places in the park.
I would suggest that you get online several times a day on the Disneyworld website (where you make dining reservations) and see if you might be able to pick up a cancellation at either Cinderella's Royal Table at MK (there is just something about eating in that castle as well as seeing the princesses while there), or Akershus in Epcot. Cancellations do happen, but in my experience, it takes quite a bit of persistance to find one. Both restaurants have Princesses at breakfast, lunch and dinner. My DD is 5 too, and she just loves those meals. If she is a Cinderella fan, you might also try for 1900 Park Fare's dinner at the Grand Floridian. The other restaurants have multiple characters, but 1900 Park Fare is just Cinderella characters. You will see Cinderella, the Prince, and the Stepmother and Stepsisters there. DD likes that one too as she is a huge Cinderella fan.
Good luck!