If you are going to DC with kids, you have to do the zoo! It was amazing and FREE! (Did I say, "Free?" Yes! It was FREE!!!)
They have 7 little lion cubs right now and we must have spent an hour just watching them ... they were so adorable and so fun! We were all enthralled with them!
Also, with kids, you have to do the Metro. Even if you have a car, make up an excuse to take the Metro somewhere. It is clean, efficient and easy to navigate ... very user friendly. It never occured to me that it would be such a kick for my kids, although I should have thought of it since we live in an area without public transportation. They thought it was a blast!
You can even take the Metro right to the zoo ... that's what we did.