The Atlanta History Center also has some awesome historic homes--The Tullie Smith House and the Swan House.
The Tullie Smith House was a very memorable field trip for me when I was little. The house is from circa 1840, and often they'll have old timey demonstrations like wool dyeing or candle making.
The Swan House is more of an opulent setting from the 1920s, but just as cool. It's probably not as interesting for kids, but I enjoy the details and architecture of old homes.