Looking for things to do in DC?

You have quite a few options depending on your interests.

First, Union Station is a beautiful place in and of itself full of shops and restaurants, so you'd rather spend your time playing in DC than finding a restaurant, then you can easily grab quick food either before or after your 6 hour stop.

The National Postal Museum is right next door to Union Station, so that's really easy. You can enjoy the station, shop, eat, and spend some leisurely time there. (http://postalmuseum.si.edu/)

You can WALK to the United States Capitol building from Union Station. It isn't too far on a nice day. You may even have time for a tour if you book ahead.  You can also stop and see the Library of Congress - a quick visit here can take as little as five minutes or as long as an hour, but it's beautiful. The Guttenberg Bible is here and Thomas Jefferson's library. They always have really interesting exhibits upstairs. Near the library is We, The Pizza - a great little pizza place. Just west of the Capital is the mall lined on both sides by the Smithsonian museums, but distance can be deceiving. You CAN walk all over the city, but taking the metro may save you time if  you want to go to a museum.

Since Union Station is both a train stop AND a metro stop,  it will be easy for you to jump off the train and take the red line. If you're interested in seeing some of the museums along the National Mall, take either the Archives Metro Station or the Smithsonian Metro Station stops (these are on other lines, so you'll have to transfer - it isn't difficult - just see the map for details). http://www.wmata.com/rail/maps/map.cfm

Your son may enjoy the National Air and Space Museum or the American Museum of Natural History. The National Museum of American History has some really interesting exhibits too.  You can check out their website (http://www.si.edu/Museums) and see what special exhibits they might offer.

The museums are all free and amazing, but check your watch. With 6 hours, you can have lunch at Union station, get to the Capital or mall area, see 1-2 things depending on how quick you are, and then head back.

It would be possible to take the Smithsonian Metro Station exit and walk to the Lincoln Memorial and see some of the other DC monuments, but that takes you a bit far away and you may have to taxi back since there aren't any metro stations really close.  It's probably safer to stick with the capital and mall areas.

www.unionstationdc.com

I agree that the National Air and Space Museumis a good bet, esp for your son.  You can always use Capital BikeShare and bike to your destination.

For food and little bit of history, you can always walk to Eastern Marketand grab something to eat there before heading back to your train.

Tiffany covered everything beautifully - the only thing I'd add is that if you go to the National Air and Space Museum - which I would recommend, as it's one of the more unique DC museums - I'd walk from Union Station rather than take the Metro. I recently moved back to the area from New York, and my friends who came down to visit were pleasantly surprised by how close the east end of the Mall is to the train station. Also fyi, the nearby National Museum Of The American Indian has a neat cafe.