+1 for the Kenndy Center recommendation.
The DC Zoo is not bad, and the walk to it from the metro is nice.
Really one of the best things about D.C., in my opinion, is the abundance of very good ethnic restaurants. Pretty much any type of food you want to try you can find an upscale place catering to it.
I don't know how old the kids are, but if they are over 12 or so and you are mildly adventurous eaters I cannot recommend enough visiting Marrakesh (
http://www.marrakesh.us/); it is one of our favorite places to go for an evening. The meal itself will take over two hours (7 courses of culinary heaven) and the belly dancing show is usually very good.
Depending on how long you are in town for, and if you have a rental car, I would also recommend taking some day trips to Baltimore and Annapolis as well (each no more than 30-45 minutes away).