My girls dress up for the princess breakfasts. We've been to both, more than once. They wear whatever princess dress they want to -- we don't worry about who will be there. They don't have to match somebody. For us, it's more like dressing up and getting to be "a princess" for the day, and they have a blast!
Last trip, my one DD wore a Meg dress to the princess breakfast. She just loves Meg, she didn't care whether she was there or not. I don't think she expected her to be, but she just wanted to wear what she wanted to wear. She was pleased to be dressed up and she had a great time.
(I personally was a little bit diappointed when Sleeping Beauty had a hard time figuring out who she was dressed as. It was from the
Disney store and was a pretty obvious Meg dress, if you watch the Disney films. I wouldn't expect everyone to know, but Sleeping Beauty?! I don't know....)
Anyhow, she should dress as SLeeping Beauty if it's her favorite. Hopefully, she'll be there, but if not it won't matter in terms of what's she's wearing. I mean, I understand, she might be disappointed in not seeing her, but it won't be odd that your daughter wore a Sleeping Beauty dress....if that makes any sense at all.
Hope that helps!