Considering the fact that Kanon seems the most well adjusted of the children and knowing this author she is the most likely suspect so far. Her reaction to hearing that the guy confessed was weird too
If he actually was the culprit, then it would make sense why he kept Kanon around despite him being an “elite” detective. They continuously say that abused children have scars, so that means there is a dark side to the detective that we don’t know