I'm guessing the story on Tench's kid is put there to make fans wonder what his future is and it matches the shows subject matter. Is he a potential serial killer. Adopted kid with an unknown past. He has some form of behavioral or social disorder. And the whole thing with the dead toddler is a very strong trauma at a very young age. Now his mother and father have issues, it's largely because of him, and Nancy is starting to show some resentment towards him too. Abusive/resentful mothers screwed up several of their killers. The kid's path is looking similar to some of the serial killers they've interviewed.
Maybe Tench and his family will resolve their problems and have a fairly normal family life. Perhaps their story will be what never happened for the serial killer at those crucial life defining moments. Or maybe we'll see Brian torturing small animals soon.