We just got back from celebrating our daughters 5th b-day at WDW... If you are staying on-site tell guest relations at the front desk that your child is having a b-day while you are there. (note: they said at least 24 hours notice cause they need to go to park and get the signature) I did and they asked what her favorite character was.. When we got back to our room that night there was a signed personalized card with a picture of Ariel and three balloons.. I also left a tip and a note for mousekeeping and they made "animals" out of washclothes..
We also bought her a tiara from our local Party City that had a b-day cake on top with 5 candles that said b-day girl.. She wore that to MK and got tons of attention!!! Even got to ride a couple rides twice. You can also visit guest relations at the park you visit that day for a free button and a b-day phone call!
Where ever you decide to go for dinner that night should be able to do something special for his b-day. We went to Tony's at MK and they announced her b-day and she got a sundae..