The museums of the Smithsonian are all free. How old are the little girls?
The American History museum has all the dresses of the First Ladies as well as Dorothy's ruby slippers and other "pop" history stuff that is fun.
I brought my 9 year old niece, along with 2 boys last year. My niece loved the American History museum, the boys loved Air and Space (she liked it as well) and they all got a lot out of the Holocaust Museum (there is an entrance fee for that, I believe, but the basic tour is fairly inexpensive and more suitable for young children than the full tour, due to the subject matter).