I've done it twice in 7 months. Sigh. (rematch). It's a pain in the butt. But doable. It's just a visit to the DMV, but you have to go with to certify she/he lives with you and is a DC resident. Both times I had to take a half day off work to get it done. Once it's done they get a temporary copy of the impending license, which allows them to drive immediately. And then the card comes two weeks later.
Now both of my au pairs were transferring licenses already gotten from other states. And they both told me they had to take the driving test when they first got their first U.S. driver's license, so I imagine that's also the case as well in DC, if they're in their first year.