Off topic, but make sure ASAP you contact your congresscrittr and senators' offices..especially if you voted for them..LOLtell them you're bringing your two kids..they love that stuff..you may get a tour pass, or a bump on the WH tour, or an invite to some event..even asked to drop by the office for pics..doesn't hurt..
I'll second the contacting your congressman. We received tickets to a "private" tour of the capitol. You arrive at least an hour before it is open to the public, learn lots & lots and it is quiet(er).
The Metro system there is wonderful and really easy to figure out.
Those who are coming in summer might want to get a ticket from the Congresscritter to see the Friday evening dress parade at the Marine Barracks. It is NOT necessarily a safe neighborhood but once one has gotten into that area there are so many people and military that it is safe during that period.
For boys there is hardly a better thrill.