I would stay outside the beltway somewhere, and take the metro in - or you could take Amtrack in and not drive at all.
Amtrack and the Metro puts you right in on the mall - into union station
and you could take a taxi or bus to local restruants or use the hop on / off "old town trolley" to get around the mall.
I was there a few times but it was years ago, and stayed with brother outside the beltway.
didn't really try the food scene with a 4 year old....
try the WP "going out guide" for food - http://www.washingto...estaurants.html
and vistors guide - http://www.washingto...tors-guide.html
Remember, Arliington NC is just across the bridge at the west end of the Mall.