I have to echo the other natives; go light! This is probably the trip that would really justify a 17-5X 2.8 lens! Long shots are going to be relatively rare, and you can probably pre-plan them, museums may or may not allow flash, and can be somewhat dark. Pretty much all of my good shots have been with my 15-85, a few with the Sigma 30mm.
Keep in mind you will WALK everywhere! Yes, we have a great Metro, but there's no Metro stop for the Washington Monument, Lincoln Memorial, or Jefferson Monument; you'll get off at Smithsonian with a few thousand of your closest friends and WALK. And, if you're going to the Smithsonian Museums on the same day, you'll WALK from one end of the Mall to the Capitol and back. I've done this many times; it is not a short walk.
Enjoy the city, get off the Mall into the Penn Quarter and have a nice meal, and if MLB season is still going while you're here, catch the Nats. A vacation with family versus a photography event, from your description!