What we have is an original series [72 to 77] and a return series [91/92] - Wiki:
The first three series' were produced for the ITV network between 1972 and 1977, with two more being commissioned in 1991 and 1992. The series was remade in 2020, again produced for ITV, featuring an entirely new cast and set of characters.
The television series was based on the characters and atmosphere, but not the plots, of the original novels. The stories mostly take place in and around Amsterdam, where Commissaris van der Valk is a cynical yet intuitive detective.
The last two series were in wider aspect, with an older Van der Valk of course and a 2 hour running time per episode after Morse had set the scene for this sort of thing. This particular episode:
Van Der Valk investigates a series of murders, including a fall, a burning, a drowning, a dog attack and a shooting, all echoing stories from Struwwelpeter. He discovers that all of the victims attended the same village school during the Second World War and links them all to a tragic death during the Hunger Winter of 1944.
First thought was that they all did it, not unlike in Orient Express, second was that one abused victim of the time did all. What's clear is that all were reticent - maybe the secretary is the toddler who witnessed it at the time, though think not because of the age. Seems more likely they were experimented on by this doctor. We shall see. Halfway through - the finding of the book alters it.