We have done two birthdays now at OKW. I have called the desk and asked if they could do anything for the birthdays. My kids got balloons with a birthday card autographed by their favorite characters delivered to the villa on the birthday.
Make sure do you do a character meal for your birthday celebration; usually they bring a cupcake with a candle for the birthday person and announce the birthday to the restaraunt. We did my DS's at Teppanyaki in May, and they brought out a small cake, enough to give everyone a piece at our table.
If you are going to do a birthday privately and are searching for a cake, I would not try Olivia's. They were going to charge $48.00 for one which included balloons, plates, etc.(I asked if it served a lot of people, she said around 5, then I asked for that price if Mickey delivered it himself!). Get a cake at a grocery store!