You didn't say where in GA the point would be to try to compute being "equidistant", so I looked at Atlanta to Boston.
I'd go with Virginia Beach, since you have three under three. DC is fun for older kids. There is a good zoo, but most of the museums and other attractions aren't going to be interesting to that age group. Same is true for Colonial Williamsburg, which is four hours south of DC, unless it's changed since I've been there. Busch Gardens is nearby, and that might be more fun for kids than the Colonial Williamsburg attractions. The Outer Banks is a long trip from MA (over 12 hours), even though the mileage is about equidistant if you compare Boston and Atlanta...the last part of the drive is very long coming from the north.