When all the family have the same surname I'd address the envelope 'The Smiths' or 'The Jones family'.
But what should I write when the adults have different surnames and I can't remember whether the kids took his name, her name or double barrelled.
I don't like using Mr and Mrs/Ms and I think 'Sandra Smith and John Jones and family' is too wordy. And I don't think its my place to double barrel them if they haven't decided to go that way, so 'The Smith/Jones family' is out.
wwyd?