Hello Deborah, we love celebrating birthdays at Walt Disney World. I am sorry to hear you are having trouble booking a dining reservation. Luckily, there are 2 other places you can try to land a spot at.
1900 Park Fare at Disney's Grand Floridian is an all-you-care-to-enjoy buffet. For dinner, Cinderella and Prince Charming join your to sign autographs and take pictures. Often times you will also see Lady Tremaine, Anastasia and Drizella. Dinner is the only meal at this restaurant you will see Cinderella, so you will want to make sure you are only looking at the dinner availability. You can also look into getting a dining reservation at
Akershus which is located in the Norway Pavilion at Epcot. This character meal is also all-you-care-to-eat and characters join you for breakfast, lunch and dinner. Princesses you may see at this restaurant include Belle, Jasmine, Snow White, Sleeping Beauty and Mulan. I very much enjoy the breakfast at Akershus, there is something for everyone. Keep in mind, characters are subject to change without any notice.
If you click the links I provided you with, you will be taken to the restaurant's section of the website. From there you will be able to check the availability for your specific dates. I also wanted to let you know that Be Our Guest is not a character meal. It is a wonderful restaurant but if it is a princess meal you are looking for, you wont find it there.
I hope you can land a dining reservation for your daughter's birthday. Have a great vacation.