You can also order birthday cakes from the hotels. They are pricey, but reports have been very good from those who ordered them.
Here are a few threads about them:
http://www.disboards.com/showthread.php?t=1822515&highlight=birthday+cakes
http://www.disboards.com/showthread.php?t=1909458&highlight=birthday+cakes
Call the front desk of the hotel and ask about the cakes.
Love the idea of renting a cabana. How wonderful that PBH will decorate it for you.
When we rent one, it is usually one of our kids favorite days. But for a change of pace, we always stay at PBH but rent a cabana at HRH. That way they are so close to the parks that they come and go all day.