Planet Word, The People's House, and the Postal Museum. Also I haven't been to the Spy Museum, but I've heard it's not the best for younger kids due to lots of reading. YMMV.
There's also a Parkour thing at the National Building Museum right now, I think, but I don't know details or recommended ages. I think they have a permanent or semi-permanent exhibit about children's book settings that I also haven't made it to.
I'd ask my kids what history they want to see. One of mine really likes the monuments.
This
https://nationalmall.org/content/horse-stables also looks cool, but it was closed unexpectedly when we tried to go so can't speak to it.