With little kids, I'd suggest focusing on the free Smithsonian attractions of Natural History, American History (Wonderplace and/or SparkLab would especially appeal), and/or Air and Space Museums...I'd also suggest the Air and Space Museum in VA over the DC one b/c you can watch the planes come into Dulles from that museum (which is cool for all ages). If you stay on the VA side, that's also an easy drive over for the day. Also, the Smithsonian Zoo, the Sculpture Gardens, and the US National Arboretum could be nice and would break up museum, museum, museum...
I'm not sure they'd love the White House/Capitol/Supreme Court/Archives...so I'd probably pass on those kind of attractions.
I might do the Bureau of Engraving and Printing b/c seeing money printed seems to be fascinating no matter the age.
I might also try to do something on the water...the Tidal Basin Paddle Boats could be nice if you come when the weather is good (they have boats for 2 and 4 - might be nice to have 1 adult with the littles and 1 with the teen)...it would be $48 for an hour without a deal (one $30/hour boat and 1 $18 hour/boat)...
I'd also do something for the kids - you could also do a VA Waterpark (Nova waterparks are pretty cheap and easy to get to and navigate), area mini golf, some of the regional parks, trampoline parks, Tyson's Corner Mall and shopping - check Groupon and Certifikid...just something so you are not just doing learning, learning, learning all the time...kids just sometimes need the decompression...
I'd also probably have you and the teen spend a night doing something without the littles...a Kennedy Center Show, a different theater show, one of the "Top Chef" restaurants, an escape room, ziplining, even a baseball game (if the littles hate it)...something so they are not always having to go by their siblings pace and so they know you planned for them to specifically have fun, too...
It can totally be fun and work for kids of all ages...just takes a little planning...
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