We just celebrated my son's 5th birthday at Disney on March 26th. Definitely pick up a button from City Hall, (or at any of the other parks, I am not sure where at the other parks).
Make sure at the character breakfast, that you mention it once you get there, to the hostess and to the characters as they come around, Mickey made a big deal out of it and wrote Happy Birthday in his autograph book. Chip brought him out a cupcake and the tables all around sung to him.
We then went to Epcot (I had picked up the button at MK the day before his birthday, because we were starting at Epcot that day).
We were facing a 75 minute wait for test track when Bill, a cast member running the lines, noticed my son's button. He then asked how old Ben was and if he wanted a birthday present. Ben got quiet and shook his head yes. Bill asked how many in our party and I told him, there were 7 of us in line, but 8 total. Bill then gave us baby swap passes (3 at 3 people each) and told us to go through the exit. Six of us rode the first time, birthday boy included! We really did need a baby swap, so the last pass the person that did not ride the first time got to go, plus the birthday boy again and his big brother! Our entire time waiting and riding was cut to about 25 minutes. Usually with baby swap passes, you have to wait for your entire party to go through the line once, before you get to go. It made our day and my son's. He thought it was great that having his birthday let him ride twice.
Not to mention that he did get wished Happy birthday all day long from cast members and even other guests.
I also the day before had stopped by our front desk at the Poly and asked what else I could do to make his day special. The staff asked me what his favorite character was and I told them Buzz Lightyear. When we returned from the parks around 5pm, there was a birthday party all set up in our room. Balloons, cake, candles, hats and noisemakers, sat on our table, plus a personalized picture from Buzz! (no charge appeared on our bill for this)
When we went for dinner at Kona Cafe, the 4 year old with us, mentioned it to the hostess that it was Ben's birthday, our server brought at the end of the meal a creme puff with "Happy Birthday" written in chocolate on a plate.
What a magical place to turn 5! You just have to mention it several times and keep asking. The button really helped too. Good luck, I'm sure your daughter no matter what age will have a great time!