I hope you enjoy your trip to DC. I enjoy the museums, but I understand that the kids would get tired of too much of that. There is the National Zoo which is nice and just North of Richmond is King's Domion theme park. King's Domion's webtite says they open April 1 for the 2006 season.
http://www.kingsdominion.com/
It is 100 or so miles South of DC, but you might stop there for a night or two on your way to or from DC.
I know of a couple of RV Parks in the DC area one is in VA here is a link
http://www.traveltrailervillage.com/
It is nice, but does not have too many extras, another is
http://www.cherryhillpark.com/
I have not stayed at Cherry Hill, but I understand it has a lot to offer for the whole family. Cherry Hill is more expensive, but might be worth it for you with childern.
Enjoy.
Bill