I have done the Cinderella breakfast twice before. I have also done the Princess breakfast in Norway, as well as the Cinderella dinner at 1900 Park Fare in the GF. Basically, all the food is the same.
If your daughter just wants to eat with Cinderella, the dinner is an option. If she wants the castle and Cinderella, then get on the phone 90 days from the day you want to have the breakfast. If you call 90 days from your trip, it won't do anything for you. Have faith and if you don't get it, call everyday. We are going to Disney in July, and I just called thed other day asking (as a joke ) if there were any cancellations. She told me no cancellations, but she did have availability for all 6 of us to eat so I booked it.
In my experience, the Princesses spend a lot of time with the kids no matter what seating you have, so get whatever time is available.