Definitely beware of the weather. It will likely be VERY hot and muggy in July.
If you are there on the 4th, the fireworks on the mall are unbelievable. Also, I think the FreedomFest is on the mall during all of July (or something like that). It has food and cultural booths from 3 different heritage groups (they did Appalachia the last time I was there).
The new(ish) visitors center for the Capital is very cool, as well as all of the museums and monuments on the mall of course.
The Metro in DC is very nice, and can get you probably anywhere you want to go.
One thing I didn't see mentioned here is the Holocaust Museum. It's definitely a favorite of mine, it is very well done.
The Holocaust Museum is very informative and well done, but if you are going to take an 11 year old and two 8 year olds there prepare to do some explaining. A lot of what I saw was uncomfortable for me, as an adult, to see, let alone for kids that age. An important part of history and perhaps a good teaching point, but maybe not something for 8 and 11 year olds to see.