Hi Sarah,
How much fun to take your little girl to Cinderella's Royal Table for her birthday! Dining at Cinderella's Royal Table is truly a magical experience.
There is no particular dress code for Cinderella's Royal Table. You will be welcome there in any clothes that you wore to the Magic Kingdom. Often, little girls enjoy dressing up for the occasion in clothing befitting of a princess.
As for special attention for her birthday, I do suggest that you get a complimentary Happy Birthday button at Guest Relation for your little princess to wear around all day on her birthday. Also, be sure that you indicate on your dining reservation that you are celebrating her birthday. Although neither of these things guarantee any special treatment, sometimes birthday girls will receive some extra pixie dust such as a special birthday cupcake. In any case, it is really fun to hear other guests and cast members alike wishing your daughter a happy birthday on her special day.
As for a cake, Walt Disney World can work with you to make a magical cake for your daughter's birthday. There are a wide range of cakes available at a variety of costs. I suggest that you call the Disney Cake Hotline at 407-827-2253 to discuss the options. The unofficial site disneyfoodblog.com has a very good set of frequently asked questions about ordering cakes at Walt Disney World. Once you get to the main page at disneyfoodblog.com, search for "cake" and you should be able to find the frequently asked questions page. Finally, you could also consider the "Her Royal Highness" package which includes a princess-themed cake. Information on this package is available on the page for
Cinderella's Royal Table.
I hope that this information is helpful for you, Sarah. Please do let us know if you have any further questions!