We celebrated my daughter's fifth and son's eleventh on our June 27-July 10 trip. (Also my mom's and BIL's, great trip!)
Definately let the front desk know at check in. I didn't say anything at SSR, but my son piped up when we were putting charging privelages on my room key. "Hey mom, since it's my birthday and all, can I have charging privelages too?" The CM heard him and started asking him questions, he told her it was his and his sister's birthday that week.
Next day we returned to our room and found two balloon bouquet's with two autographed pics. Kids were sooo excited, and my family was impressed.
Same thing when we moved over to POR, only I think the Magical Gathering planner had it noted on our ressie.
We got the birthday buttons at each park and the CM's were great. My son was given Front of the line passes for Buzz by a CM in Tomorrowland. He was given pins by many, many CM's.
Rosie in MGM sang to him, then handed him two pins she found in the "sewer" while cleaning up. He thought that was hilarious.
We had it noted on our PS's too. My daughter was sung to at the GF Tea and served a huge cupcake.
My son got a great big sundae at Sci-Fi, and he had to stand up on his seat so the restaurant could sing to her.
People would stop us just to say Happy Birthday or sing to the kids. It was great.
Finally, they both got birthday phone calls from Goofy.
I also had made t-shirts for them to wear on their special day, with a baby pic and Happy Birthday printed on them. Embarrassing, yes, but great pics of them wearing the shirts.