The same tip that is important for the Nat'l Archives is important for the Spy Museum as well. There are also two lines at the Spy Museum; the one to buy tickets, and the one to get in after you get the tickets, because the tickets are timed.
So, decide when you are going to the Spy Museum, and buy the tickets online ahead of time. The sooner you buy them, the better the pick of the time you want (and yes, you can buy tickets for March now.) BTW, it's good to do the Archives and the Spy Museum on the same day, as they are located very close to one another. (And close to Chinatown. I *really* recommend having dinner in Chinatown; the food there can be extraordinarily good.)
When it comes to kids and the Nat'l Archives, the treasures room usually doesn't do that much for them. What they DO tend to like is the part at the end of the tour, where you are turned loose with a lot of very ordinary documents on exhibit, with audio commentaries to listen to about what they are. My kids were interested in things like ration tickets from WW2, Civil-war era letters, draft cards, etc.
Hands down, THE place that we found was a favorite was the Bureau of Printing and Engraving (the factory that makes printed money). Tickets to this one are EXTREMELY tricky to get, as it is very popular and cannot normally be reserved in advance unless you are touring with a group, or have gotten tickets way in advance by requesting them from your Congressional Representative's office.
This tour is free and is only available on weekdays, and again, times are assigned. Same-day tickets may be obtained from the kiosk next to the Holocaust Museum on Raoul Wallenberg Place. The trick is that they open the kiosk at 8 am, and by 8:30 all of the tickets for the day are usually gone. You *must* line up by 7 am if you want to be successful at getting tickets during a busy week. What we did for this one was send my DH as a scout: each person in line is allowed to claim up to 4 tickets, and we needed 4. He likes taking photos, so he got up before dawn and took his camera to the mall for sunrise shots, got in line at 6:30, got the tickets for an after-lunch slot, had breakfast, took some more photos, and we met him at our morning destination. Here is the link that explains things:
http://www.moneyfactory.gov/wdctourtickets.html
BTW, I would say don't bother with the Capitol Tour unless you get passes via your representative's office. The wait to get in is long and complex, but you only get to see two rooms on the general tour. The sponsored tour is MUCH better.
I recommend doing the monuments in the evening after the museums have closed, but unlike most folks here, I don't recommend doing the "Monuments at Night" bus tour; very pricey, and the bus is uncomfortable IMO. I think that one of the best ways to do them is by bike. There are formal bike tours with guides, but again, they are on the pricey side. The cheap way to do it at your leisure is to use the Capital Bikeshare system, (
http://www.capitalbikeshare.com/home). The system has public bike rental kiosks in DC, so you can take Metro to the nearest one, rent bikes, pedal your way around to the monuments at your leisure, then return the bikes to a kiosk and go back to your hotel.