Quite an overgeneralization, IMO! We (those in the U.S.) do stage protests on occasion, some quite large and often multi-city. And we use other fora to express our opinions publicly. I can understand that people (whether from the U.S. or not) might think we don't protest various issues often enough or clearly enough, but misrepresenting our actions is not likely to serve anyone well, IMO.

I agree with Gretchen's post upthread: DC is, IMO, a vastly underrated city. IMO, it offers a wealth of opportunities for people of all ages and with a wide range of interests. If you haven't already done so, I would recommend consulting a good guidebook, one that covers not only the sightseeing options, but one that also provides you with a sense of the depth of their coverage so you can plan your time with confidence.

DC is a great city to visit and all the Smithsonians are free. Lines will be longer in the summer months and the heat and humidity will be terrible. I say May into early June, kids are still in school in the US. Avoid Memorial day weekend in may. September and October are also great months because all the kids are back to school after labor day in September. Late September would be an ideal time to visit.
The election is in early November.
There are always people out and about protesting something or campaigning for something in and around the the White house and Capital area.
There is so much to see and do! Enjoy!

I would say your best bets for both crowds and weather will be either early-mid May or October. We were just there last week during my kids' mid-winter break, which many area of the US do not have. The museums, zoo, monuments, and Mall were the quietest and easiest to navigate of any time we've ever visited.

As many people mentioned, the summer can be VERY uncomfortable there, as well as very crowded.

KayF, the museums are excellent and they are FREE!