I'm going to buck the tide here re: the Monuments by Moonlight tour. It is VERY expensive, IMO, and not all that pleasant, especially with younger kids in tow. My recommendation if you want to do the monuments at night is to use cabs to do it. Cabs in DC charge on a zone system, so they are relatively cheap, and it is generally easy to walk between the monuments; the strenuous part is getting over to the monuments area from the nearest Metro, which is where the cab comes in. On most clear evenings cabs are easy to hail at the major monuments, but plug a cab service number into your cell phone, and you are good to go at your own pace.
I also think that the standard Capitol tour is not worth the hassle of going through all of the security and waiting that is required, because you really only get to see two rooms: the Rotunda and the Sculpture Hall, along with the visitor center. If you really want to
see the Capitol, stick to the tours that are only available by writing your representative in advance.
I second and third the Bureau of Engraving and Printing; it is a GREAT tour, and you can only do it on weekdays; they are not open on weekends. The same-day walk-up ticket booth next to the Holocaust Museum opens at 8 am, but if you are not in line by 7 am you probably won't get any tickets. Each person in the line can claim up to 4 tickets, so unless your family is large, you can send one early-bird scout to pick them up. DH is our scout; he is a camera-bug, so he's fine with getting up at the crack of dawn to go fetch them and take morning-light pix along the way.
There is no charge to get timed-entry tickets to the Archives, and it also is a good tour, but you need to reserve your entry time well in advance. You can do it online at
http://www.recreation.gov/tourList.do?contractCode=NRSO&parkId=79052
BTW, one privately-owned museum that has not yet been mentioned is the one at the Nat'l Geographic Society. They host some REALLY good travelling exhibits there, and it is worth checking out what they have going on for your dates.