M.M.
Just a warning the Holocaust Museum is very intense. I would not recommend it to preteens and younger. It really freaked my daughter out last week and she is 15, History class field trip. My son went at 16, years ago, field trip, and he too said not to take the younger ones.
You can get out of the city and stay in Fairfax, Springfield, even Stafford, Woodbridge, MAnassas. The hotel rates are cheaper, but then there is the traffic to contend with, our traffic is worse than Chicago's or anything in Indiana. I think we rival LA. Take the METRO, it isn't too expensive and a great experience, and will get you anywhere downtown.
Take the METRO to the zoo. Do not try to park near or at the zoo. It is almost impossible. THe METRO station is about a 10 minute walk to the zoo. And the zoo is a must do in DC. That will take about a day. The zoo is free, parking is not.
The sightseeing tours are a really good place to start. THey stop at the monuments and all the museums and you can just hop on and off all day. They also go to Arlington Cemetery.
Some places require tickets, the Capitol, Washington Monument, Mt Vernon. You can get them online.
Williamsburg, Jamestown, and Yorktown are about 2 1/2-3 hours southeast. THat would be a great side trip, as would Baltimore or Annapolis. You would want to stay there. Busch Gardens is in Willimasburg too.
Fredericksburg is a quaint, antiquey town. It also has Ferry Farm, George Washington's boyhood home and a fantastic German restaurant, although very expensive. The Bavarian Chef, it is very authentic.
Food is crazy expensive downtown, but it is fun to do the museums and the monuments and then grab a hot dog from the vendors on the mall. Sometimes the experience is worth the price.
Oh, King's Dominion is on 95 South of DC, exit 98. It's an amusement park like Busch Gardens. That's only about an hour or so away.