We liked the Isle of Palms beach the best too. Folly beach wasn't bad - but the water was a little too rough for younger kiddos... and there wasn't a storm coming or anything that would typically make the currents bad. I got the impression that Folly beach was more of a 'surfer' type beach (although having lived in southern CA for a few years, I have a hard time picturing lots of surfing any Atlantic beach). If the water hadn't been so much rougher there, we might have gone back - we went to the state (or county?) beach that had the lifeguards and little concession place and it ok, the current and the rough water just made me uncomfortable when hubby would take our 5yo out into the ocean.
We went to the state (or county?) beach on IoP too - you do pay admission but it was well worth it for us to have the lifeguards, bathrooms, etc. They have a little playground too which is nice for the younger kids. I don't remember a concession stand there though.
Of course, if the OP is going soon, I don't think he is interested in the beach info - might be nice to take a little stroll along the beach and get some pictures - but no need to worry about the water.
OP, if you are going to be down there for a bit (a week or so) and you get 'bored' you could always take a day trip to Myrtle Beach. We have family there and went down for two days while we were in Charleston - took rt. 17 straight down and it took about 2 hours or so to get there, so you could definitely leave early in the morning and get back late at night. My oldest could live at Broadway at the Beach - with that little wizard place where you get some sort of electronic wand and find stuff, the IMAX, the dragon putt putt, the aquarium, the rides, and Joe's Crab Shack (both of them just love painting the crab shaped rice krispie treat) he is in heaven when we go there. Youngest is getting old enough to enjoy a lot of that stuff too. And if you have kids into Harry Potter or wizard stuff, that place where you buy the wand and go around on different quests is pretty neat, I just can't remember the name of it off the top of my head.