We just visited DC with the kids over spring break. Hotels in downtown DC were very expensive that week- like $300+ for a Hampton Inn. We ended up at the Embassy Suites Chevy Chase Pavillion, which is right on the DC/MD border. The friendship heights metro station runs right under the hotel and it was very convenient to get around. It is only 3 stops to the zoo, and about a 15-20 minute ride to the mall/museum area. The rates were much more reasonable, and you get two rooms, which is nice for kids. They also had a free hot breakfast which saved time and money in the mornings.
We had a wonderful time, but I don't think I would go during spring break again. The lines and crowds were far worse than I imagined. I felt like we spent half our time in line- it was hard just to see exhibits because people were 4 deep pushing to get close to the glass. Then again, I hate crowds so my tolerance could be way less than yours.