I don’t blame the victims for not telling. And it is possible that this monastary served as a residence and school for children who did not have family. Or if they did the family was most likely unable to care for them. Think of what conditions must have been like in post WWII Germany and you get an idea of how difficult it would have been to report the abuse and for authorities to investigate it. Even if they had been inclined to do so.
As to why the monks who were not part of the abuse did not report it, I can only shake my head in sorrow.
Generally this would be quite plausible except for the fact that the 1960s and 1970s was a rebellious and openly anti-authoritarian period in both US and European history. That serious abuse of this kind and extent would go completely unreported, not even to any of the parents or any of the friends and neighbors or to any civil authority of any type is highly problematical and at least intuitively impels a suspension of belief.