We just celebrated our DS9's birthday in October at WDW. When I made our PS's, I asked them to note it on each PS. We had a few magical moments.
We had a character breakfast on a Sunday at 1900 Park Fare. Alice in Wonderland and Bert from Merry Poppins were there. Both made a big deal about DS's birthday -- singing a very off key Happy Birthday to him. DS was given a birthday cupcake as well as a birthday placemat.
That same evening we had PS's for LTT at 5 pm. Our family was asked to be the "Bell Family" of the evening. DS has the opprotunity to welcome all the guests and open the restaurant for dinner. I'm sure "we" were selected because it was noted on our PS that DS was celebrating his birthday. I also ordered a wonderful whipped cream cake at LTT ($10 extra) when I made the PS. It was absolutely delicious! The characters came over to wish him a happy birthday.
Another evening we dined at Cape May. The CM brought DS a birthday cupcake and acknowledged him by name. When DS questioned how he knew his name, the CM said it was just part of "Mickey Magic".
On an impromptu note, we stopped at Crystal Palace for lunch. The CM went out of her way to acknowledge DS's birthday based on a birthday pin he was wearing. DS received a birthday candle as well as a birthday card signed by all of the characters.
By far the most magical moment we had because it was DS's birthday was a trip to the roof of the Wilderness Lodge -- which is where we were staying. We had hoped to be able to do the daily flag ceremony but the days were already filled when we checked in. DH found a CM who was able to get an exception for us to be taken up to the roof explaining that it was our DS's birthday trip. It was certainly a breaktaking experience.
Many CM's wished our DS a happy birthday based on the free pin he received. One of the CM's at Animal Kingdom dialed the phone in her store to have my DS speak to Goofy and get wished a happy birthday. There are many "surprises" that can happen (DS did not know I had a hand in any of them

). I might be in trouble, though, because DS3 is already asking to have his birthday at WDW. I'm not sure I can duplicate the "magic" twice.
Have fun!