Was just in DC 3 weeks ago. Would recommend the Westin Washington. They cater to families with a great milk and cookies program (free cookies and bedtime age-appropriate book for the kids), as well as great frontline staff that really look after the kids. They also have a daily listing of activities and events in the city that they provide everyday. $40 valet parking is a bit steep and it's a long walk to the main sites in DC proper. Priyanka Potnis - wonderful front office manager.
Also stayed at the W the same trip. Great hotel and stay and was really looked after with a great view of the city from the hotel rooftop (White house/Washinton Monument/National Mall/Treasury building) and a close walk to everything you want to see. Only unfortunate occurence during my stay was that someone decided to pick the W to jump off the roof of the hotel... Sad.