Every time I go into D.C. I find myself in the movies. D.C. is the backdrop to so many movies and T.V. show your almost guaranteed to find yourself on the set of a movie you know. Our second stop of the D.C. day with my parents took us to the set of National Treasure. Okay not really but we were in the National Archives which is featured in the show. Even if you have never seen the movie I'm sure most people know that the Declaration of Independence is on display in the Archives, making it a popular D.C. stop.
Having previously been to the Archives and having Callen in the stroller (getting up to see the Declaration of Independence there are 3 or so stares making it not very stroller friendly), I viewed the Maga Carta while my parents at the Declaration and Other documents. The Maga Carta is a facinating peace of history in which the Declaration is based on (I think, I'm a biologist not a history major). In addition to the Declaration and Maga Carta there is also a gallery which has an exhibit on Discovering the Civil War: Consequences. This exhibit does a good job of showing how the Archives are used to find new information on the Civil War. It was very educational but not that stimulating (if that makes sense).
While the National Archives are interesting and have lots of cool historical documents this is not the most toddler/baby friendly stop in D.C. Unless you are a history buff or are kidless then I'd skip the Archives and head to the American History, Natural History, or Air and Space Museums as they are much more fun for little kids.