We went this past summer and had no idea what to expect.
From the outside you couldn't tell this was a "Mansion" with a million rooms inside.
The afternoon tea I would not say is what you'd expect usually when you go to "tea" at the Ritz or Willard or even in the U.K.
It's a buffet assortment of things ranging from cakes to cookies to PB&J sandwiches to vegetable platters with ranch dip. I definitely saw some families and I think its a kid friendly place for sure--not fancy, but eclectic with tons of stuff.
My boyfriend calls this place cluttered but I found it fascinating, everything in there is from a certain event in history and each ornament and figuerine or newspaper clipping told a story (at least to me hehe).
Anyhow, here's some pics but I'd recommend it if you're looking for a place to bring some kiddos along too or something outside the normal afternoon tea experience :)
The Mansion on O Street
2020 O St NW, Washington D.C., DC 20036