Myrtle Beach Travel Park has an indoor pool. It will be "splashing" weather more so than swimming weather most likely. We did the Dixie Stampede for dinner one night and it was a lot of fun. The NASCAR go-kart track will be fun for the 5 year olds (and probably dad!) plus it has other kiddie stuff there. Ripley's Aquarium is not huge and perfect for all the kids. Barefoot Landing and Broadway on the Beach have lots of amusements, shopping and one has an alligator park. There's endless miles of white sand beach to walk as well.
I'm a big Gettysburg fan but at this point MB is a better choice.