South Florida to South Dakota and Wyoming and back to FL

High Falls State Park just south of Atlanta is one of our favorite stopping places. Since you have 12 days I'd suggest a stop at Great Smoky Mountains followed by Mammouth Caves. We generally prefer to avoid St Louis since they have awful traffic and worse road surfaces but you may want to do a stop there to see the Arch, the kids will like that. From there to South Dakota is something of a empty spot in our experience. Not a lot of things to go and see until you get to SD.