Here's the first iteration of "the Double-Double", as I called it.
Trek T1000 with kidback stoker, tandem trail-a-bike no pedaling. Lead trail-tandem child pedals are aligned and zip tied to be footrests. Caboose position had a rack-post to allow child seat with full harness.
This evolved through eldest child as Rear Captain, shifting gears for two girls pedaling, and son on the kidback. At that point the total weight was making things unsteady, even by the standards I had gotten used to.
It was always kind of a novelty thing, but being able to ride 15 miles to the park with three kids and a picnic, doze under a tree while the kids played on the climbing structure, and take the short 7 miles home with happy exhausted children really saved my sanity a few weekends.
Now eldest does 25mile/2.5kft rides with me, and is pretty gracious about this being the summer she can beat me up hills. She's asking about how far she can go by herself, too.
I still have the tandem. It's time to take the kidback off it and take one of the younger kids with me on some medium rides, see if I can spark that interest.