Hi Samantha! I have spent many a Walt Disney World vacations with my multi generation family. The memories that I have from each and every one of them are priceless and I hope we have many more to come. It is truly the best destination in the world for all ages to enjoy and find magical.
The
Hoop Dee Doo Musical Revue is one of the most popular dinner shows on property. It will be a great spot to celebrate your nephew's ninth birthday with your family. They will certainly acknowledge his special day by announcing it to the rest of the guests. Between that and all the dancing and music it will be a fun evening.
As far as a cake, I would suggest you save it for when you are all together back at your resort. You can order something special and take your time enjoying it at your leisure. The dinner shows have a fast pace and there are shows back to back so you can't linger in the dining room once it is over. I would hate to have you rush through eating and you would likely have to box it up and take most of it back with you.
You can order cakes through your resorts or you might enjoy trying something different. The new pastry shop,
Amorette's, in Disney Springs, has adorable MIckey ear hat cakes. They are reasonably priced and are delicious. They come in many different varieties beyond the typical Mickey ones so you do have some great choices. They do not deliver so someone would have to run over there and pick it up but you can order it by phone so it will only be one extra trip instead of two.
I know whatever you decide it will be a most memorable birthday for your nephew!