I've been to Coral Reef several times. It had neither good food nor a good atomosphere for younger kids. That being said, it delivers a cool experience. I don't think this experience will satisfy a six-year-old for long, and the rest of your meal, you'll be stressed out.
A character meal at Crystal Palace or Chef Mickey's will blow the mind of any kid. Both of these will deliver food (while buffet, a style I don't care for) that will usually be as good as the choices you made, and an atomosphere more in line with what a child of 6 is looking for.
That being said, brace yourself. There will be many other young children there. But then again, you can let your son be himself on his birthday without concern.