If it were me, I'd spend a week in Williamsburg and a week in the DC area, unless you've already spent some time in DC or have other plans to go there separately at a later date. It's a long day trip from W'burg and there's so much more to do in DC.
I agree with paxsarah's suggestion, one week in Williamsburg (Marriott Manor Club or other) and possibly try to reserve Wydham Old Town Alexandria (must try to reserve this early though for busy summer season) for a week.
You can easily spend a week in D.C., plenty of attractions to visit. It is about 3 hours or so travel from Williamsburg so it is better to stay in D.C. for an extended period to make it worthwhile.
Williamsburg is great for families. As others have indicated, there is also much to do there but about a week or so seems just right to me for your first visit.
Recommend visiting the Jamestown Settlement, Historic Jamestowne, Yorktown Victory Center and so much more. Many great dining options all over the area too. Be sure to try to schedule a "parent's only" night out for a special evening together at one of the area fine dining establishments.
Include a day or two at Busch Gardens (and maybe King's Dominion?) and nearby Water Country to mix the educational with fun and thrills for the easily bored with the historical (but most of it is presented very well, quite interesting).
BG is a very nice, clean amusement park, well done, lots of roller coasters there too if that suits your youngsters.
Research on your own and check out the many posts here on TUG BBS that have weblinks and comments. Reserve early as summer is peak period for family tourism there. Have a great trip.......