To me it sounds as if each boy gave his version of what he saw happen.
Student or Learner
Each boy said what he thought had happened.
This sentence is taken from Collins Cobuild Usage, without context. The Chinese version of the dictionary translates the sentence like this:
Each boy said that the thing that he had thought of had happened.
But I would think this sentence means:
Each boy described what had happened the way he thought it happened.
Could you please explain what exactly this sentence means?
It means, as you say, "Each boy described what had happened the way he thought it happened."