Our DD turned 7 during our trip. Don't wait until the day of her birthday to get the birthday button, get one for every park and wear them everyday. The CM's go out of there way to say Happy Birthday, and even the characters in the parade will sometimes come over and shake hands. My DD was so happy when Belle mouthed it to her from on top of the float during the parade. They are told to look out for those buttons, that's for sure.
She even got a couple of free pins from CM's because they saw her button. Don't feel weird about wearing it every day either. Toward the end of our trip CM's were still saying it and I would tell them that her birthday was a couple of days ago and they all said, "Everyday is your birthday at Disney, so enjoy it!"
Go and have a great time!