Riddle,
While I'm Jewish and can't really help with the Christmas present problem, I did just celebrate my daughters 5th birthday on board the Sensation this summer, and if you tell your waiter ahead of time (her birthday was the first night on the ship, but he still managed to pull it together) s/he will round up all the waiters/waitresses and sing to your daughter. We ordered a cake from this website, and it was GOOD, like really, really, really good. And relatively inexpensive (I think it was $7... much less than I would have spent at a bakery here in Philly) and they put five candles in it, lit them and all. It really was memorable. From the time we boarded (we had the VIP check-in and she saw on my daughters BC that it was her bday and she must have alerted whomever because everyone knew, and I sent her off with my parents after we boarded and I managed to get to the room WAY early to take decorations on the door [double sided tape is a MUST have] and the room steward brought a whole crew of towel animals to put on her bed, and they really went above and beyond. After we got back from dinner there was a cookie face, and more towel animals, and she had colored a little birthday card (in both English and Thai, and that was REALLY cool).
So yea... if you take the little bit of effort, you can make her birthday MEMORABLE. my daughter is already bugging me about which ship we're going on next year for her birthday and if that can possibly be topped (looking at a disney cruise, but any ship for her would do)