If you're interested in Lincoln, Ford's Theater is very interesting. You can tour the theater, get a ranger presentation about John Wilkes Booth, the political situation at the time, the assassination and its aftermath, and you can go through the fascinating museum in the basement.

It sounds like a great trip for both you and your grandson!

The tourmobile has great on/off service for the monuments. You can buy their package that includes Arlington, which is a must-see in my opinion. You can also take the metro there if you want and just purchase the cemetary portion of the tourmobile tour. It drops you off right at the Kennedy graves and the tomb of the unknown soldier.

The Capitol is a must obviously, if you your grandson is into politics. There new visitor center makes it much easier to visit than it used to be. You can schedule a tour online or with your Congressman. I recommend your Congressman, but make sure to request passes to the galleries, because they are not inlcuded with the standard tours. Sitting in the galleries during a session is pretty fascinating for those who like politics. Also, keep in mind that there is a tunnel to the Library of Congress from the Capitol visitor center. You can easily walk there without going outside or going through security. (You do have to go through additional security if going from the Library to the Capitol, since the Capitol is stricter than the Library). The Library of Congress is another must see in my opinion, especially with their new exhibit of Thomas Jefferson's library. There are a ton of fascinating historical documents on display there.

Since you want to limit walking, be sure to take advantage of cafeterias in the various museums and buildings. It may not be the best food or cheap, but it will allow you to save your energy for the sights. My motto in DC is always "I can eat when everything closes." (Although my co-travelers don't always appreciate my travel philosophy.) The Capitol has a great cafeteria. The Supreme Court has a smaller one, but it is easily accessible. Union Station has a huge food court with a ton of fast food type options. Most of the Smithsonians have food options as well, although they are pricey.

Mt. Vernon is pretty awesome. I went in November and didn't want to leave. I think it is PERFECT for a nine year old. That may be the ideal age to visit.

Have a GREAT time!

Also check out the DC Circulator bus system that links all the tourist sights, sometimes more easily than Metro.

http://www.dccirculator.com/

Be sure to check out the special Smithsonian performances. They do a lot of kids' programming.