I agree about the KOA--it's close to Mt Rushmore and there's a lot for the children to do...pool, horseback riding, etc. Also, they have a restaurant, a snack area--pizza, ice cream, etc. and they serve an inexpensive pancake breakfast in a huge tent.
Just to mention--the whole family might enjoy Bear Country USA a few miles away...a drive through park with lots of bears and wolves and other wild animals. It ends with a zoo of baby animals.