Welcome to DIS!
I have had two birthdays onboard the Disney Wonder. Both times they (our serving team) brought me a little cake and sang to me at dinner. I just asked DH how he arranged that.
He went to Guest Services once onboard the ship. He told them it was my birthday and that he wanted the small individual cake delivered to our table on ____ night. Guest Services puts it into the system, which also let our dining room team know. And the rest just took care of itself. He also confirmed with our Head Server to make sure he was aware of it. He was.
You can also pay to have a room decorating package put up in your room, but with it being a small kid, you could probably find a chance to decorate the room yourself when she's not in there at some point.
Hope this helps,
Dreams