They don't promise (or guarantee) which Princesses will be in the room at any given time. We were anxiously hoping for Ariel and Belle (note my name, lol) but even as we stood outside waiting they couldn't tell us who would be in there because it was almost time for a shift change. BUT....as my Belle loving daughter grew more anxious about whether Belle would be there, the CM did say that Belle normally is at the photographer because her gown is hooped and very difficult to move among the tables. Cinderella apparently had her hoop reduced so she fits better, but Belle's dress didn't accomodate that option.
We got lucky and both Ariel and Belle were there. Belle taking photos, and Ariel circulated once, and then was not seen again for our dinner, although we did see all the other ladies come around several times, so I think we totally lucked out and caught Ariel before she left for a while.
I was prepared for a few tears from my girls if their favs weren't there, but then also reassured them that if we couldn't see them at dinner, we'd still get to meet them. Ariel is in MK in her grotto almost all day (she takes breaks, but otherwise she's there.....but in her mermaid outfit not a dress/legs). Belle can be found most days in France in Epcot, though not in her fancy yellow gown, but in her Blue and White town dress. Belle in her yellow dress can also occasionally be found in Toontown, but the CM there said Ariel doesn't appear in either fins or legs.
If you ask any of the CMs at a particular park they can tell you when and where you can find any character. When we've asked and that particular CM didn't know, they'd call and ask someone else....never did I have a CM say "I don't know"....the worst we got was "sorry that character isn't here today, but so and so is". So definitely ask!