Are you still planning to stay in Interlaken? In your other post it was recommended by people to stay in Wengen or somewhere in the mountains, which makes a slight difference in this answer.
I recommend AirTime from Mürren, which means you need to get to the Schilthornbahn cable car station in Mürren to the meet up point, hike 15 min uphill with the crew to the launch point, fly for around 20 minutes, land in Stechelberg and then I recommend going back up to Mürren for an afternoon of activities there - Schilthorn/Birg Thrill Walk, Allmendhubel funicular + alpine playground, hike or walk depending on abilities (possibly Blumental Flower Trail from Allmendhubel or walk from Mürren to Gimmelwald)
If you’re trying to do this with kids, I’d probably recommend everyone going to Stechelberg, one adult rides the cable car up to Mürren to the meet up point while the others stay at the landing site (the field by the Stechelberg station parking lot). You’ll probably wait around 45 minutes to an hour before the person lands (have them text you what color their parachute is so you can be watching for them). Others will be flying and landing all the time if it’s nice out, so you will be able to watch various people. Once they land, either the family goes up together on the cable car to Mürren and the other adult wanders around Mürren with the kids while the last adult goes and then meets you back in Mürren (there is a small playground across from the Sportzentrum) or family stays at the Stechelberg lot to watch the other person land. To do this in stages is probably going to be around a 3ish hour commitment.