I just got back from a week in DC with the family which includes kids age 10-18. Overall it was a simply wonderful trip. We are Arizonans and most have never been East of Chicago. This is just a bunch of random observations.
Humidity. Temps were 80s, but it felt horrific coming from altitude in lovely AZ.
Walking. We had a Residence inn close to the Smithsonian and walked/metroed everywhere. I think we racked up around 50 miles on our feet in 6 days, which the kids tolerated well since DC is both flat, and sea level, two things our home town is decidedly not.
Safety. DC felt safe, at least in the mile or so surrounding the Mall and most of the tourist areas. Constant security screenings were in place and generally handled well. Federal LE and armed private security were pretty much everywhere. I found it amusing to see a private security guy heading home with an empty holster. I talked a bit with a couple private guys and they were absolutely okay with leaving the gun at work. The guard actually said: "Some people just can't be trusted with guns." Guns were sometimes revolvers, lots of G19s and some M&P. Secret service had a compact metal Sig (assume .357 Sig), Supreme court police had Sig 229. Interesting fact: The Supreme Court has its own police force which is the only police under judicial authority. Small, 145 members, very nice and professional. Generally I felt safe even completely disarmed and with a bunch of kids in tow. Only one crazy bum cussed out the oldest son which is pretty decent for the miles we covered.
We did the tours of all three branches of government, saw the court room, saw the Whitehouse, saw the Capitol. Met our congressman. I think I enjoyed the Capitol the most.
We did tons of museums. All at taxpayer expense. Air and Space, Natural history (don't miss the gem collection which really caught me off guard), and National Archives were stand outs. Wife loved the National Botanical Garden.
We hit tons of the memorials, including the changing of the guard at Arlington, which impressed my son a lot. I saw no Sig Tomb pistols (or any pistols) with the Tomb Guards, only M14s. My favorite memorials are actually MLK and FDR which are right next to each other. Both are very inspiring given the speech excerpts they chose to put on the walls. FDR in particular is extremely relaxing and really under rated as the big memorials go. But we still did Lincoln, Jefferson, Vietnam, Korea, WWII, etc.
Overall our favorite building by far was Library of Congress. We spent well over half a day exploring the place and its exhibits.
Metro was great and easy to use. Landing at Reagan is less then 30 min by walking and metro to the hotel. I never regretted not renting a car.
There was more we could have done, but overall, it was an outstanding trip and I recommend it for kids of all ages, but probably best 12 and up.