Busch Gardens is a bit away from the other major Williamsburg attractions. You could get a hotel close enough to walk to the colonial area, but you will still need to drive to all the other attractions in the area. There may be hotel shuttles.
And IMO Busch Gardens is not that great for younger kids. We went last summer when Amy was not quite 3 and there were relatively few things she could do besides the Sesame Street area. It's not really like Disney (IMO) where most attractions can be enjoyed by kids of most ages.
There isn't anything like Callaway Gardens close to DC.
There are full service resorts in the Virginia mountains. The Homestead is lovely. Not a lot for really young kids IMO and very expensive.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)