IR cannot give a definitive answer about actually seeing little DeOrr when they were all heading to the creek after returning from their trip into town. His answers are ambiguous and confused. IMO IR was led to believe that DeOrr was alive and well at that stage but I'm leaning towards little DeOrr being already deceased and disposed of by then.
IMO whatever happened to little DeOrr happened during the course of the Thursday night or the early hours of Friday morning. Nothing has been mentioned as to whether any of them left the campground during the night but I think that is highly likely. If something happened to DeOrr in his parent's vehicle or close by, and one of them left the campground in the early hours, the other would know. How could they not know? By the same token IR and RW may not have heard a thing if they were sleeping 'heavily'. I'd personally like to know what VK. JM and RW were doing when IR went back to bed and slept till midday.
In a nutshell I don't think that IR has a clue as to what actually happened to little DeOrr and is only telling it like he remembers it. And there is no descriptive memory of him seeing little DeOrr heading to the creek with his parents.
PART 2: Isaac Reinwand describes what happened when DeOrr went missing.
https://youtu.be/hrzdY1ck3Zk