One of the top places to take kids in DC is the Botanic Garden. Open 7 Days a week (except Christmas Day) this place is spectacular! If you are visiting with younger kids, especially if you and they are confined to a hotel room, very soon you'll need to find a spot where they can be free to roam that's also free of charge with a close-by spot to warm up (or cool off in summer) and find restrooms.
The Botanic Garden is all that PLUS the place is full of cool live examples of spectacular living plants and phenomenal toy trains - both inside and OUTSIDE. Trains Outside? Yes- the Botanic Garden has a very large outside area - fenced - with lots of levels and lots of interesting large scale insects! On the west side of Botanic Garden's glass headquarters outside of the main building you'll walk (or run) in fresh air though small scale models of farms from around the globe! Through these tiny outdoor farms a large scale (but still small) railroad runs- complete with trestles, and cliffs and lakes. The cool part is that there's nothing much to break and no alarms to set off if you bend over to peek into a tiny railroad station. And don't forget that family shot - in front of the holiday tree.
Now inside the Botanic Garden - wonders are all around. You know those monuments you came to see? Each of DC's famous monuments is displayed in one of the Botanic Garden's big glass chambers but here, instead of being made of white stone, these small scale replicas are made of plants and natural fibers. Imagine the Washington Monument at 7 feet tall made from wild weeds and leaves! And of course there are even more trains winding their way up above in your family creating new toots and choo-choo sounds to join the bird song in each of the three vast micro-climate areas you'll be strolling through.
As an added bonus the Botanic Garden is about 3 blocks from the Orange Line- Capitol South Metro. From there walk towards the US Capitol - the Botanic Garden and outdoor area is nestled on the west side of the Capitol at the bottom of Capitol Hill.