The National Cryptologic Museum is one of DC's most fascinating attractions. It's run by the National Security Agency (NSA) and showcases how cryptology has been used throughout U.S. history. It's actually the first and only museum in the intelligence community!
Here, you'll have a great opportunity to learn about how the NSA uses codes and encryptions to communicate in times of war and crisis. It goes as far back to slavery, showing how slaves used codes and signals while escaping. The National Cryptologic Museum also offers incredibly informative tours, and usually the tour guides are NSA employees. You can call ahead and let them know if you're coming with a group of kids, as they have tailored children's tours as well. Parking is really easy to come by, so you don't have to stress about finding parking either.
The best part about the National Cryptologic Museum is admission is free. They're open most days except for Sunday, so try to plan your trip accordingly.