They have a replica of the Auschwitz gates, which say “Arbeit Macht Frei” (work makes you free) across the top. I hesitated before I walked through them. The cattle car also stirred up a powerful response- I believe that was from one of the camps.
I’ve been there twice; the first time really wigged me out. The second time I steeled myself beforehand and came through it more even-keeled. My spouse had the opposite experience. I think it can be hard for people of any age to grasp what really happened, but the nice thing about the museum is that they try to tell the story in a lot of different ways. I think it was the pile of shoes that got me the first time around. I am sure everyone has a different thing that blows them away.
The first time we went, there was someone at the start of the museum reminding people to pace themselves. I didn’t see that the next time, so I don’t know if they generally do dispense this advice, but it’s useful. There’s a lot to see.
There certainly is a lot to see. It’s the kind of place I would like to go on my own, if I can find a day without the kids.
What was exactly what my 8th grade History teacher said blew him away too. He said that through all his traveling, visiting historical landmarks and museums, it was honestly the most depressing place he’d ever been to in his entire life. 
For a time, I got panic attacks in large crowds. The one exception to this was when I went to the museum. I had a hell of a panic attack. It was moving. . .but. … yeah. . .wow. I’d like to see it again, because I think I missed a lot of it due to needing to leave. I really, really freaked out.
I don’t think it would be from MacArthur. He spent the war in the Pacific ocean, fighting the Japanese. I don’t think he was ever in Europe or saw any of the camps until afterwards.
Maybe it was Eisenhower?
From http://64.233.167.104/search?q=cache:o3Z_aXa6V2IJ:www.scrapbookpages.com/USA/USHMM/Exterior.html+"holocaust+museum"+plaque+washington&hl=en&ct=clnk&cd=7&gl=us The photos on the page are pretty well done, too.
the OP asked about a 13 year old, but I’d like to add a suggestion for the parents:
Don’t just look at the museum exhibits–look at the reactions of the other visitors standing next to you.
The exhibits are great–they touched me deeply, even though I know the details of the Holocaust.
But was was truly more meaningful to me was to see how the other visitors were touched. When they blinked the lights at closing time and told us to head for the exits, NOBODY moved. Dozens of people, all strangers to each other, but everybody was absorbed in his own serious thoughts/emotions. That’s when I realized how effective the museum is at teaching, and I hope, at keeping history from repeating itself.
The Japanese camps weren’t much better. I have a friend who survived Changi.
Just one addition to the Metro information. There is no charge to park on weekends. Parking downtown will indeed cost you your firstborn, even on a weekend. If you can find a place.
Native almost Washingtonian.
This is probably too late but for logistics: If you’re coming from Harrisburg, you’ll be coming down I-270 (15 to 270) and there are Red Line stations that are close to your route. Probably Shady Grove is the one most convenient from your direction. The Greenbelt station is on the Green line - and that’s a longer drive for you (you’d have to go east on the Beltway). You’d need to change trains at Metro Center or Gallery Place to get to the lines others have mentioned.
The museum itself: I have been only once, when Dweezil was a toddler (went because friends were in town and wanted to see it). It was very, very powerful. I could not see the whole thing because my son was melting down (he had no clue about the place, he was simply acting like any toddler who was sick of being in the stroller). I will go back, some day. Perhaps in a couple of years - Moon Unit is far more aware of things than Dweezil is and I think it might be good for her. Some day.