We did this for our kid's birthday a few weeks ago. Just call Coral Reef directly and get it set up, and then remind them when you arrive. One extra benefit is that you'll get a tank-side seat. The divers will swim down with a laminated 'Happy Birthday xxx' sign, and give it to you afterwards.
Here's a dark picture (it's hard to get good pics in there). The stuff in front of the diver on the right is 'confetti' - He grabbed material from the bottom of the tank and tossed it up as part of the celebration.