Belle is always there; she is always at the entrance for the photo op, and she is always in the gold dress, except around Christmas, she is sometimes in her red holiday dress. There will always be four additional princesses who visit the tables and lead the princess parade. The current official line-up is Ariel (in a green ballgown), Aurora, Snow White, and Cinderella. However, should one of them be unavailable for some reason, Mary Poppins or Mulan are the current alternates. On our recent trips, we had Mary Poppins instead of Aurora one time, and Mulan instead of Cinderella the other trip, I believe. Both our Mary Poppins and our Mulan were absolutely fantastic, and we were happy to see them, since they are slightly rarer princesses at the meals.
Don't guarantee a child anyone other than Belle. There was a little girl crying in an Aurora dress, and I heard her mother berating a cast member for promising Aurora and not delivering. Made me really sad for the child, and the mother who had expected Aurora, and the cast member trying to explain that they aren't the same every time.