Imagine, if you will, Mick Jagger performing "Satisfaction" at John Lennon's house, where John and Yoko record him for posterity.
This is the kids' music equivalent. Elizabeth Cotten performing "Freight Train" at Pete and Toshi Seeger's house, as recorded by the Seegers. The sound quality isn't the best, and I've noted this clip before, but now it's been posted to YouTube. Given how little video we have of the early days of recorded family music, this is quite valuable. (Not to mention a great song.)
Elizabeth Cotten - "Freight Train" [YouTube]