Definitely keep checking for CRT openings. Spots open up all the time, but it's better to stalk the dining part of the website for openings vs. calling. i.e. since it's one thing to check the website 30/50/100x per day but it's not really practical to call like that. While not the castle, Akershus at Norway in Epcot is a princess meal, and there is a Cinderella dinner at 1900 Park Fare that's also an option. Just be VERY flexible with your times/meals in all of these cases. Not sure what the avail. is on this one- but perfectly princess tea over at the GF could also be an option.
Make sure you request a bday button when checking in and have the bday added as a celebration to ALL of your reservations. Many of the restaurants will do special things on someone's bday and WDW staff also like to wish people wearing bday buttons happy bday. DD5 even had a bday serenade at a QS spot in Epcot (Sunshine Seasons).
You could also consider a cake. Best place to get info on that is to check out the cake chatter under the restaurants section. Prices will vary based on what you want. If you can get into CRT, they even have a handful of special pkgs- one of which involves a princess themed cake.
We also got DD5 a special bday outfit for park touring on her actual bday and used crystal stickers (candles and alphabet) to decorate princess mouse ears.
While I think you have a better chance of getting into CRT over this one-- you could see if BBB has any openings. You have a better chance of getting the DTD location. If you can't get into BBB, I've found it fairly easy to replicate the experience with a princess gown, fine grade glitter/hairspray, tiara comb, and a little bit of makeup/nail polish.
There are just so many ways you could make a bday special. This is just the tip of the iceberg, but hopefully it helps.
