Found by strangers helping on Twitter. Never mind, mods feel free to delete if this is misplaced.
Hi, is there some special phone number that @crschmidt (on Twitter) should be calling to get United to help find his unaccompanied minor?
To quote, "My daughter's flight through IAD got delayed, and she's now stuck in the airport over night. @United can't tell me where she is. She's 13."
I haven't the faintest idea how to help on my own, but I figured I'd ask on Flyertalk in case someone has prior experience with this.