Colonial Williamsburg might be great for your nine-year old, but most seven and five year olds will not appreciate what is offered there. Will be a great trip when they get older. However, they would all love Busch Gardens.
As far as I know, the FBI Building is no longer open for tours, and I think the White House has only reopened tours to school groups. Your congressperson will know. If you want to go to the Burea of Engraving and Printing, you must make a reservation ahead of time to enable them to do a security check.
Please take the advice of a previous poster and see the monuments at night, as well as the daytime. There is a narrated Tourmobile that offers hop-on/hop-off sightseeing. It basically encompasses the Mall area, Arlington National Cemetary & I think possibly the National Zoo. It is a great way to get around. Do not waste your time going to the National Aquarium. It is beyond disappointing. However, the Baltimore Aquarium, about 40 minutes north of DC, is fabulous. The Museum of Natural History usually shows at least 2 Imax Movies and has a great hands-on area for the younger kids. My 5-year old was enthralled with the "Lewis & Clark Expedition" Imax Movie last year. If it is still playing, it is a must see. We also took my 5-year old to Mount Vernon. This is definitely worth the trip. So much to see; so little time.