I would have to agree with Aileen about the weather. While it is one of my favorite times for touring the city, it would not be my pick for going into the ocean. For more swimmable beaches, you may want to look further south to Florida if that fits into your timing.
I do love Kiawah but dinning options are pretty limited out on the island. The club house at Turtle Point does have a nice restaurant as does the hotel The Sanctuary. There's a place over on Seabrook Island (adjacent to Kiawah) but that's about it. Both the islands are pretty remote which of course is all part of their charm. To get to Charleston from there, it's about a 25-30 minute drive now that the new roads are in place.
You may want to think about Wild Dunes Resort (www.wilddunes.com)in Mt. Pleasant (a 10-15 minute drive from downtown Charleston). The resort would fit your needs of quiet, close-by dinning options, on the beach with the added advantage of being close to all the great sights of in Charleston that would be fun for the kids. We've got a great children's museum (www.explorecml.org) and aquarium (www.scaquarium.org) downtown...both have recieved great reviews from kids I know around your grandchildren's age. Both of these are in downtown Charleston. From the Mt. Pleasant side, you could tour the USS Yorktown (www.patriotspoint.org) battleship (little boys especially love this) and take the boat tour over to Fort Sumter (www.fortsumtertours.com)...where the US Civil War began. Also on the Mt. Pleasant side is Boone Hall Plantation (www.boonehallplantation.com)...my personal favorite plantation in Charleston. They usually have a corn maze during the fall around Halloween so that may still be up and running when you come to town. The plantation itself is pretty fun...the only thing you will not be able to do is take the plantation tram as all passengers must be at least 6 years old.
Just a few suggestions as we'd love to have you come to Charleston!