Sounds like a great trip - even the planning for someone else's trip is fun! Four teenagers and an 8-year-old does make it a bit tougher, though!

I write Civil War travel books, so I'm probably going to be a bit biased. That said...

Appomattox would be a great stop, and be sure to see the American Civil War Center's new museum there. Keep in mind, though, that it's a bit isolated and may take some extra drive time to get to. If you do it, try to loop in Monticello and maybe Montpelier, which are sort of in the same general area. Sort of.

If you want something interactive for the kids, check out Pamplin Park/Museum of the Civil War Soldier in Petersburg. You're on the last major battlefield of the war - the one that drove Lee to Appomattox - and it's very well preserved. The exhibits are very interactive and geared toward an immersive experience. If I wanted to get a kid interested in history, this is where I'd take them.

If you go to DC, take advantage of your representative in Congress! If you contact their office, they can generally have one of their staffers give you a personal guided tour of the capital (bypassing all the long lines!) and/or gallery passes, and may even be able to squeeze you into a White House tour, which is not open to the public at large. It's the only way you'll see it.

If you're interested in the Civil War, I have endless tips for you - feel free to contact me!