I hope the photoshoot went well? Many excellent tips have been shared here, so thank you everyone!
Since you intend to use manual focus, I had a couple thoughts. First, my guess is that you’ll want to use a tripod. I’ve never done a family photoshoot without autofocus, but it seems to me that a tripod would help. Secondly, I would suggest you focus on arranging the people and fixing little details before taking a shot. There are two styles of portrait photography, one is quite off-the-cuff and candid in nature, snapping lots of shots while the subjects are doing things and interacting with each other; the other being more formal with intentional posing and close attention to detail. Without autofocus, I doubt the first would be very easy to do.