I checked out the wikipedia entry and the imdb entry for "Family Affair", and they both claim that the children's names were Catherine “Cissy” Patterson -- Davis,
Jonathan “Jody” Patterson -- Davis, and Ava Elizabeth “Buffy” Patterson -- Davis . Their uncle was William ( "Bill" ) Davis, and they were his brother's children . Was there ever any explanation why their name was "Patterson -- Davis", or was this information correct ?

That information is incorrect. In an episode where they visit Terre Haute, their mother's maiden name was referred to as Patterson, but the kids were never called Patterson-Davis on the show, just Davis.

Thank you, O . G . ! I see we both live in Montana ! Where in Montana do you live ?