Lisa,
The Metro is really easy to get around on. One thing I suggest you do that I did not mention, is to call your local Congressman before you go and see what they can do for you. Ours got us tickets to several things when we went, like the bureau of engraving, and the Supreme Court. He also had one of his aides give us a tour of the Capital, and we rode the subway that the Congressmen ride (no big deal, believe me) between the Capital and their offices. They can even get you tickets to tour the White House, but I heard since 9/11 you have to go through a security check at least 6 mos in advance.
If you get a chance to see any of the other monuments, Lincoln, Jefferson, and FDR are really neat. One of the most moving for me was the new WWII memorial, since my dad and all my uncles were a part of the service in that war. You may already be aware, but just in case you did not know, admission to museums like the Smithsonian is free - you just have to pay if you do things like iMax films, etc.
I bet William will really have a great and educational time. Have fun!
John