My dd wore the button at MK on her 8th birthday. She received a few Happy Birthday wishes from CM's and other guests, and also got the phone call from Goofy wishing her a Happy Birthday while we were in one store.
I think it all depends on which CM happens to notice the button, how many other buttons he's already seen that day, and what he's in the mood to do for you. Please don't expect anything really special (like ride passes, etc), because you might end up disappointed if it doesn't occur.
To me, just being in the MK on a birthday is special enough. You'll have a great time!