The more I watch video footage the more I start to think that there was no deliberate formation or photo shoot or any other close formation work planned. Both banner towing aircraft would not be in close formation as they were towing banners of rival political parties (CDA and SP), and the Cessna would not be in a preplanned formation with any of them since it seems like it's just been a pleasure flight. At least, the young lady in the video was asked about what she thought about her first flight in a light aircraft. It might be me but I would not want to have a first-time flier with me when I'm doing a photo flight like that. And when you do a photo flight, you take pictures of the plane, not from the view outside the opposite window or your legs. And you keep up with the airplane you're photographing, instead of overtaking it. Furthermore, I cannot hear any radio traffic that would be consistent with a formation or planned photo shoot.
So it looks to me that the "formation" between the Cessna and the Cub was actually just an overtaking maneuver, although one executed in relative close proximity. In that bit of airspace you don't have a lot of choice. Sea on your right, a Prohibited area (EHP26/26a) on your left, possible opposing traffic going North over the beach, and the Schiphol TMA (class A) at 1500' above you. So the Cessna was simply overtaking the Cub, when the Cub noticed the Husky underneath the Cessna. He pointed down to indicate the other aircraft, at which point the RHS passenger saw the wing of the Husky, but by then it was too late to avoid.
The Dutch commentary of the girl and the guy at the end doesn't have a lot of substance to it. To a large extent it's just the rambling of two people who had a very close brush with death, but lived to tell the tale. The things that I found noteworthy are above.
I think the collision wouldn't have happened if the the Cessna had continued straight and level, but for whatever reason (better view?), it descended a bit - right on to the wing of the Husky.
The wind at that day was S-SE, very calm, and it was a bright sunny day. There might well have been thermals coming off the beach.
At 3:31 does it look to others as though the Cub pilot is trying to draw the photographer's attention to the Husky by pointing his finger?
The guy at the end of the video commented that he saw the Cub pilot pointing down, just before he saw the Husky. So that might well be the case.
Question arises, who took that photo of all three planes?????
The beaches were packed that day. I would imagine that quite a few of the sunbathers had cameras with them.