I took DD (just the 2 of us) for her 5th birthday in 2005 and can't wait to do the same for DS in 2008! I made her a personalized bday shirt and she got the birthday button at Guest Relations, but very few CMs/characters seemed to notice. We had a character breakfast (at CRT, natch) and dinner at Chef Mickey's on her day, and I noted the bday on both ADRs. There was a cupcake and appropriate fanfare at both locations.
I had made note of her birthday when I first made our hotel ressies too. I reiterated this upon check-in, and the CM at the desk asked me who her favorite character was. Late in the afternoon (while she was napping) on her birthday, Pop Century delivered some baloons and a card with Cinderella's photo and autograph. What a nice thing for her to wake up to!
But the most special moments were the unexpected pixie dust. I just hope my DS is as lucky when his turn rolls around. DD was very outgoing on this trip, and told just about every CM that it was her birthday (so it really didn't matter if they failed to notice the button or her shirt!). As a result of some of the conversations she got into, she received two birthday phone calls from Goofy, was given a couple of trading pins (from 2 different CMs) that she didn't have to trade for, was sung Happy Birthday to in Spanish by a lovely Hispanic CM, was sung to in English by a large group of CMs who were in training, and was deemed a Jr Chef and got to assemble her chocolate "moose" dessert at Le Cellier. The Le Cellier CMs also gave her a birthday card even though it was the night before her actual birthday.
Enjoy your trip and happy birthday to your DS!!!