I’ve never been on either side of a mini-session. It sounds a bit like dress-up night on a cruise ship, minus the ship. They set up a few “studios” off the main dining room and invite people to stop by for photos, which they then put into a gallery, hoping to have them purchased by the subjects, as mementos of the trip.
You could do something similar. Set up a decorated Christmas tree, put a few wrapped boxes under it. Place a couple of chairs and/or a couch. Have people come in and sit. Take a few photos. Have them pretend to open some of the presents — keep some extra wrap handy in case kids are too enthusiastic and tear off the wrap. Take a few more photos. Done! Tidy up and you are ready for the next session.