Coming from Central Ohio, I would recommend the following:
1. Stay at the Monroe, MI KOA and visit the Henry Ford Museum, Greenfield Village, Detroit Science Center, Detroit Zoo. All are a short drive once you get set-up at the campground. The campground is good for kids with many planned activities.
2. Drive a little further out to the Port Huron KOA. Great place for kids with more activities than they'll be able to take in. Adjacent to the campground is Sawmill City. A small amusement park with go-karts, bumper boats, water slides, etc. It is close to the Thomas Edison Museum, Fort Gratiot Lighthouse.
3. Cross the boarder into Canada. Stay at the Jellystone in Amherstburg, ON. Another great campground with lots of activities for the kids. Visit Fort Malden, Jack Minor Bird Sanctuary, Colisanti's Greenhouses, Point Pelee National Park.
Have fun!