One thing I've done for my DD's birthdays when we've been there (twice, so far) is I've brought character birthday cards with me to the parks and I've had the appropriate characters sign them for her. Then, when we celebrated her birthday in our room, the cards were waiting for her. The princesses cards were the easiest to find. I've managed to have them signed, without her knowledge, thanks to some help from the characters themselves. If there are others in the room (Mickey's Toontown, restaurants, etc.) they help to distract her.
Also, make sure you get the Birthday Buttons at the parks (from guest services). CM will wish them

all day long. As an added bonus, my DD received free desserts when she was wearing her button

.
Also, several times when my DD was wearing her button, she got a special phone call from Goofy wishing her a happy birthday. This happened both times at the Epcot, Guest Relations when we pick up the button. The CM says something like, "... what's your name. Oh, we had a call earlier here for you," and then they hand them the phone. My DD got a big kick out of this when she was 5 and 6. She also got the call from Goofy in Mouse Gears (Epcot) and also in the lobby of Hollywood Brown Derby, at MGM.
Each time we've gone for her birthday I've mentioned it when I made our room ressies, but the only thing she received was a "Magical Moments" certificate at one stay. Some posters report getting balloons, or signed picture in their rooms, but this hasn't happened to us.
Also, if you are going to be eating at table service restaurant, on their birthdays, mention it when you make your ADRs, and you might receive something special. If you want to make sure of getting something special, call the restaurant direct and make a request. You can get birthday cakes ($) at some restaurants, but my DD was very excited just to get some Mickey confetti on our table at CG last year, along with the whole restaurant singing

to her!
Hope this helps, have a great time, and

to your DD and DH!