Ooh I went a few years ago and I was really pleasantly surprised.
I spent a whole day walking around the Mall - the Lincoln monument, Vietnam War and Korean war memorials, and all the way through to the Capitol.
The Smithsonian museums are amazing - I only had time to pop into the Air & Space Museum, but it was awesome - your kids will love it. Likewise the Natural History museum - if you've been to the one in London, you'll like this, the displays are way more impressive.
The Capitol building itself was fascinating - I did a guided tour, highly recommended - and on a whim I went through to the Library of Congress. I knew nothing about it, but I was so glad I went there, it's absolutely stunningly beautiful. Well worth a look.
The National Arboretum was great too - fantastic greenhouse full of tropical plants, with periodic 'rain / mist' to keep them watered, which was fun!
All of this was within the strip known as the Mall, where you can just wander around all day.
I wished I'd had time to go up the Washington monument - the giant needle, apparently you can get tickets to go up it but they sell out each day.
Have fun!