I've never had extended family at the interview....that's just weird, there is no reason for them to be there.
From now on I would try to work it in the initial emails or phone calls something along the lines of "I look forward to meeting you and your child" or something like that. I'm not sure is there is a tactful way of saying that anyone else is not welcome.

Definitely, if they mention anything about bringing extra people, I would politely let them know that the parents and dck is welcome at the interview, but anyone else will have to refrain from coming, as it is just too distracting to the interview itself.