I am a little biased because Washington DC is one of my favourite cities, but I really think you'd be hard pressed to find a city in the US as beautiful and with such a variety of wonderful cultural sights, one the highlights of which is the Air & Space museum. I have visited Washington DC twice and this has been a highlight both times. There is a great variety of exhibitions, both historical and scientific and plenty to interest children. Apollo 13 and the Spirit of St Louis are amazing to see, but remember to look up as you come in!
There is a food court of fast food and the service is very quick and the gift shop is very extensive.