Hi there, we did the same thing on our last trip, in December 2007- a one day beach trip. We went to Cocoa Beach, and had a wonderful time. It was ds (he was 4 at the time) first time to see the ocean. The day we were there the locals warned us that there was a "red tide," so we shouldn't swim, but we saw other families splashing in the water, so we went in, and never had a problem.
The sand at Cocoa Beach had lots of small broken shells on it, which made ds feet sore, so maybe bring some sandals or water shoes while there. Also, I felt like the water was a bit rough, so I only let him wade to his knees. Since it is on the Atlantic Coast, the water is definitely not clear, but we found the beach in general to be pretty clean.
We are going to wdw again this August, and we decided we are going to try a Gulf beach this time- after researching, we are going to try Siesta Key, because reviews say the slope into the water is very gradual, and the sand is very soft and doesn't get hot to the touch because it is mostly quartz. Also, the gulf water is much clearer, which makes dh happy.
Let us know what you decide- I am sure it will be fun whichever beach you choose, and it is nice to have that one "down" day to just relax.