1. Use Metro - it is important to realize that DC is a public transportation city - a car is a waste of time and a big cost. Get a Metro card and load it with $$$. I would travel a few times with a kid who has never really done the subway or bus thing. A few stops and explorations and bing they have it. If they have a bike, bring it - but there are lots of hills in DC. Also get a really good lock (and take the front wheel with you or locked as well.)
2. It Gets VERY Hot - DC summers are brutal, so make sure where ever they are living they have AC or a big fan (AC is almost a must). Make sure they have the right type of clothing to wear (if you are on the hill you will need a tie, button downs, blue blazer, kaki pants as guy and something similar for the women - just make sure it is COTTON!) - Also make sure they have a reusable water bottle and if possible a way to filter the water. They need to stay hydrated - but in the DC area a reusable water bottle is better received.
3. A Fan For Your Desk - I would get them a little clip on fan for their desk - while most building have AC in many it is not that cold, more like cool. It will help.
4. StarBucks - I would get them a reloadable card - StarBucks fuels the capital.
5. Security A Way Of Life - Always build in time for security checks, you have no idea on what day and what time the lines to get in ANY building in DC can be a breeze one day and two hours later it is a 45 minute wait. Also do not be surprised by all the police. In DC there are over 65 different police forces (really 65) - from the Capitol police, to the DC police, to the parks police and so on. ALWAYS have your ID with you - always.
6. Give Your Kid A Helping Hand - I always like to get delivered to the kids desk a big order of cookies and juice, big enough to share with those they work with. Even an edible arrangement is cool. Send it to the desk not the apt. Embarrass the heck out of them (secretly they will be thankful.) Don't ask (they will say heck no) just do it.
7. It Costs More - No matter what you think they need it will always cost about 30% more in the DC area. Just make sure you have a way to get $$$ them fast and easy. There are ATMs every where. While the Smithsonian is free - everything else costs.
8. A Southern City - DC really is a southern city - people are friendly, always feel OK asking from help from police or anyone in a uniform. As in most cities of the south, things take time and go at a different speed.
9. Traffic UGH! - 15 minutes can make all the difference, 6:45 vs 7AM can make your travel time on the Metro much nicer, same at the end of the day staying a bit longer at your desk vs leaving with all the others at 3:30/ 4PM can make the ride home better. Starting a 7AM the traffic into DC ramps up to the peak at 9:30AM (that is why I try and have 10AM or 10:30Am meetings - it only takes 30 mins to get from Northern VA to DC vs 2 hours sitting in traffic) and by the end of the day starting at 2:30 the traffic out builds up until 6PM. Just be aware of the crowds of people coming and going.
10. Airports- Most people know of Dulles (which is a $50 to $70 or more cab ride to down town based on the day, time, etc.) and also National right down town (which has a metro stop at the airport). There is also BWI (Baltimore International) and it pretty close to DC and a short cab ride and the air fairs can be cheaper.
11. Checking Up - If you have a friend in the DC area make sure you introduce your kid to them and share the contact information. Ask them to stop by and take them to lunch or to an early dinner and look them in the face and make sure they are OK.
Hope this helps, at the recent Washington Post special section on "the intern's guide to DC" is great tips too.