You can get a birthday button at your resort or any park guest service location.
We took our 5-turning 6yo to Chef Mickey's for his birthday. He absolutely loved it.
- They get a Mickey faced cup cake and special laminated birthday placemat; my nephew uses his daily at home now
- They take a photo of your family and add a Birhday burst to the bottom to mark the occassion ($30 for the photo package)
- The food is plentiful and good with a lot of variety
- I like to call the atmosphere sanctioned chaos, it bright and loud with music playing, napkins waving and the monorail cruising overhead
- There are 5 characters so plenty of interaction and photo ops (Mickey, Minnie, Pluto, Goofy and Donald Duck)
- CM's is in my nephew's top 3 things at Disney (Characters, Chef Mickey's and Goofy's barnstormer rollercoaster)
We canceled our ADR for this other place, but it might also be a great choice for you: Play 'n Dine at Hollywood and Vine (Breakfast or Lunch). Characters from Little Einstein's and Jo Jo's Circus make appearances there. There is lots of music and dancing here.
I would not order a cake at the restaurant. There is plenty of dessert on any of the buffets and the birthday boy will get a cup cake. That way you don't have to worry about waste or carrying the cake in the park or back to the room. If you really want to do a cake at Disney have it at the room so you can store the leftovers more easily. I must admit I kept trying to come up with a reason to order one of those podium cakes, but never could justify it!
Good luck with your plans.