Thanks for the info offer, vakamalua! I do have a couple of questions (which can probably be answered by our guides when we get there, but I like to plan ahead...

).
I'm mostly concerned about safety, especially when we are "on our own" during and after the tour. We live 30 miles SE of Seattle, and go "into town" only occasionally with the boys, so they lead a pretty sheltered life as far as being aware of their surroundings, and as cities go, Seattle is pretty tame.
Just use common sense as you would in any big city--be aware of your surroundings, don't carry around any valuables you don't need and secure your wallet/bag especially when in a crowd or people are close to you (elevators, Metro, cross walks, etc).
We do plan to have a car after the tour is over (even though it's $45/night to park at the Mayflower!!

) but I'd like to ride the subway in DC if we choose to go much farther than a mile or two. We have tix to the Evening Parade at the Marine Barracks on Friday night; would it be safe to ride the subway there and back, or should we take the car? I know we'll look like tourists no matter what, but I don't want to take any unnecessary risks.
The DC Metro serves all the tourist areas very well; compared to most large cities, it is safe, clean and easy to navigate. Driving and parking in DC can be very difficult so try to take the Metro when possible. I don't think you'll have any safety problems using Metro to get to and from the Marine Barracks; it's about four blocks from the Eastern Market stop and the area is full of little shops and cafes.
There's a GREAT local place to eat on Capitol Hill that you might want to try before or after the Marine Barracks--'The Good Stuff Eatery' owned by Spike Mendelsohn (from Top Chef). The best burgers and milkshakes in town (try the Toasted Marshmallow shake-YUM). Spike also owns the place next door, 'We the Pizza' so your party can get whatever they like and still share a table in one place together (there's also limited outdoor seating). Your kids will enjoy it. It's about half way between the Eastern Market and Capitol South Metro stops so you wouldn't have to backtrack (bonus: walking by the Capitol at night).
http://www.goodstuffeatery.com/
Also, I requested White House Tour tix many months ago, but just found out we didn't get our request (which isn't too unbelievable as it was for the Saturday after the 4th of July

). The "you didn't make it" email had info about a White House Tour Center, or something like that - do you recommend we spend the time to go there instead?
I would make a last minute pleading call to your senators and congressman's office before you leave, just in case. With so much to see and do in DC, I wouldn't bother with the WH Tour Center, not much there.
And speaking of the 4th, I'm hoping that Disney has something special planned for us, if only finding a great place for us to watch some fireworks; if they leave us "on our own" as we're supposed to be that night, what do you suggest? My kids are so bummed that they won't be home to shoot off bottle rockets and I keep telling them "what could be better than to be in Washington DC on the 4th???"
I find it hard to believe that ABD would leave you on your own on the busiest night in DC, so I suggest verifying that with them before you leave. If you are indeed on your own, definitely plan to see the 'not to be missed' fireworks. Security is much higher on the Mall than it used to be so here's my suggestion: as soon as you check-in (or even email today), sit down with the hotel's Concierge and ask them to help you plan the best place to see the fireworks given your timing and the location of the hotel. Maybe the hotel even has a roof top they open for viewing.
So any info along those lines would be much appreciated! Thanks!!!
We leave in 5 days - lots to do to get ready!!