CMs are not nessisarily obligated to make birthdays very! special! Sometimes they are just having a hard day, sometimes they don't see the birthday button. So, if you go in to a situation not expecting anything, and if you do end up with something, you'll always be happy.
That being said, you can guarantee that there's a cake at any Disney resturant by calling a certain number. The cake runs around $10, If i remember correctly. I don't have the number on hand, but the nice folks at Disney Dining can give it to you.
If you choose not to order the extra cake, there is always the good chance of getting a cupcake, extra dessert, or birthday song, but remember it's still a chance. Nothing is guaranteed.
Personally, I say go for it without the extra cake. There's so much to eat there for dessert anyways! Or try one of the other dining options around the world! Chef Mickey's is great, but perhaps you'll get more personal service at a sit-down place? I dunno. Either way, hope they have a magical birthday!