Well to start with go to City Hall in the Magic Kingdom and get them a birthday badge ......the CMs that notice the badges will wish them a happy birthday!
You could treat them to Cirque Du Soliel, or even the IllumiNations Cruise - they also do a birthday IllumiNations Cruise. You need to call the cruise in the moment booking opens 90 days ahead - if you go to the following link it tells you about the birthday cruise and also birthday cakes which you get at full service restaurants