I didn't read all the replies to your post, but here's our take:
Gettysburg - if you & the kids are in to military you will enjoy it here. And even if they get their fill of military there is lots of other things for them to do, and mom & dad, too. We like the Adams County Winery and the Round Barn, both just west of Gettysburg off of Rt. 30 near Cashtown. Our favorite eateries in Gettysburg are Kilwin's (for ice cream), Dunlap's (basic diner food) and Gen. Pickett's Buffet (and there is one waiter at Gen. Pickett's who can regale the whole family with lot's of interesting history about the battle).
You could spend a whole day at the Visitor's Center alone looking at the weapons, uniforms and other displays. Don't miss the cyclorama. You can tour the battlefield in a number of ways; we borrowed a CD that narrated the battle in chronological order - gives you a good idea of how the battle unfolded over the three days. You can even tour by horseback if so inclined.
Colonial Williamsburg - there is much more to this place than meets the eye. It's not just static displays; take in the demonstrations, musical & theatrical programs, re-enactments, etc. The fife & drum corps parade in the late evening is not to be missed. Our campground of choice in the area is American Heritage, but we've stayed at the KOA and it is nice, too. Sites tend to be small.
DC - You can get a Metro bus right at Cherry Hill CG that connects with the trains or go back up the highway to the Greenbelt Metro station. To me, the Metro is THE way to get around DC. Cherry Hill is a nice park, but remember you are in the middle of a densely populated urban area, so there will be highway noise. Sites are small.
Philadelphia - maybe because I live so close, the thought of stopping here doesn't float my boat. Yes, it's the "cradle of the Revolution" because of the things that happened here, but over all IMHO Philadelphia is a dirty city and traffic is horrendous. Personally, I'd skip this and spend more time at some of the other attractions you have on your list.
Yes, your itinerary is ambitious, but do-able. Enjoy & safe travels.
