I would say do not bother ordering a special cake, save it for back home...you will be full...even if you don't like the food, there is a large (and good) dessert section on the buffett, including excellent soft-serve ice cream & sundae bar. (We liked the food, all except the pasta/meat dish & the rolls)
And, if she's wearing the birthday pin & you have it noted on your 1900 Park Fare reservation, chances are they will present her with the special autographed & laminated 1900 Park Fare placemat...and bring her out a special cupcake decorated with Mickey sprinkles & a Mickey candy disc on top. They did this in May 2009 for DH -- and he was so full he only ate one bite of the birthday cupcake but said it was good.