And along with that excellent advice, do not apologize for "not
knowing the content as well as they do!" If they ask a pointed
question, say something like "we're going to be learning about
that together," because, you will be learning about that together!
Also, have something nice to say about their child, and specific,
if possible - "I love the way Johnny thinks about things," "Seeing
Suzy smile at me really encourages me," stuff like that. Anyone
can say "s/he's a really good student," so include why you would
say that. That lets them know that you really do have a personal
interest in their child.
Smile, nod, take copious notes and love their child as much as
they do! And don't let them see you sweat..... You'll be great!