Make It a Full Week
You should plan on spending a full day at Mount Vernon/Woodlawn Plantation (make sure you explore the Pope-Leighey House, designed by Frank Lloyd Wright, which is also on the grounds) and then have dinner in Alexandria, Virginia. It's a delightful way to explore the past and present, and there's so much for both kids and adults.
Preteens and teens might be interested in the Washington Segway tours, shopping at Georgetown and the outlets, and visiting the National Museum of Health and Medicine, which has some of the grossest medical exhibits in the country. If your kids are over twelve and mature enough, the Holocaust Museum is a good family event, though very difficult to experience.
You might also want to consider spending a day at one of the nearby amusement parks — Six Flags America in Largo, MD, is closest. Take Metro's Blue Line toward Largo Town Center and exit at Addison Road. Transfer to the C21 bus and exit in front of Six Flags. Further afield are Paramount's Kings Dominion, 75 miles south in Doswell, VA, and Busch Gardens, in Williamsburg, VA, 150 miles south. It's a great way to let your hair down after an educational vacation.