Last August, we were at WDW for my Dad and brother's birthday (they share the special day). We did all kinds of cool things. Each park has buttons that say "Today is my birthday!" and the cast members really treat you special if you have one on, especially a child. (My dad turned 62 and my brother 39, and they still fussed over them!)
Also, plan a character breakfast on birthday morning (call now). We did Chef Mickey's at the Contemporary. They brought little birthday cupcakes and all the characters made a big deal out of the event.
Some things we did would not be open to children, like getting into a show such as the Backstage Tour water show at D/MGM, but if you are at Animal Kingdom, they would probably let you do the "hand jive" at the Festival of the Lion King. If you are at MK that day, I would recommend doing the scavenger hunt, and sit at one of the "stop" points for the day parade ( in Liberty Square, to the Haunted Mansion side of the Hall of Presidents, facing Frontierland is our favorite spot). They get kids to join in the parade. You need to have your spot at the rope by 2, though. Ask at City Hall for your birthday button and info on the hunt and any other birthday kid activities.
Have a great time!!