Just mention the birthday when you make reservations and your DD will be given a cupcake and sung to.
I have to say the cakes in the birthday buckets are not that good. We have gotten them 4 times and all four times they were bad. They taste exactly like what they are, previously frozen cake that is thawed out a little before you get it.
The same goes for the Plaza Inn Birthday Celebration. The cake they give you to decorate is sometimes still frozen in the center, GROSS!
If you wanted to buy a cake (which I would not recommend) you have to do it 48 hours in advance (cakes other then the birthday bucket). They do not budge on this 48 hour notice either. They are VERY over-priced and again, they are previously frozen cakes.
If you will have a car there then go to any of the local grocery store, you will get a much better tasting cake for a fraction of the price. Just remember that you can't bring a cake into the parks so you will have to bring the cake to a restaurant in one of the hotels or Downtown Disney. If you do bring your own cake then the staff will store it for you and have a character come sing to your DD when they bring it out. This is what we did for my daughters birthday party's in June at Goofy's Kitchen. See pictures below from Goofy's Kitchen...
Brooklyn in front of her cake
Madison's Cake
I have to say that I think that Goofy's Kitchen is awesome for a birthday but I have to say that a VERY close 2nd is the Lilo & Stitch Breakfast. We have SOOOOO much fun everytime we go there. I think the advantage to GK is that there are face characters that can interact with your children, where as Lilo's Breakfast none of the characters can speak so interaction is limited.
Hope this helps.