The version I got (this kit from Digitec) is a 2 (or 1) seater as in the first video. The last video is not from this model: there are no small loops every 4cm at the top to attach seats on this model.

I still used the Maddison in the Gorges du Verdon with my two children and my wife. The children were 4&6 and it was certainly not quite compliant for 4 hours but it worked. I would say that two adults and a child under 8 can fit if the child sits on the back/front cushion without a seat. This is only a solution for fun on the beach, no whitewater!