If the player discovers the secret of the town, by going into either the Wilsons' shed or the Smiths' basement and subsequently kills the residents, Old Man Harris will become the guardian of the children and speak more openly of his history. He is the one who tells you about the cannibalism, but the others tell you he went mad ever since his wife died.
If you kill Jenny and Junior's parents, they will come to live with Old Man Harris. If you kill Old Man Harris after this, the children do not seem to care or notice, even if you do it right in front of them.
Killing Old Man Harris will not incur a penalty to Karma.
Even after you kill Junior and Jenny's parents, Old Man Harris will continue to say things like "Don't be fooled by them!" during dialog.
"No! Well... yes. But it's all a trick. Don't you know anything? Did you just crawl out of a vault or something? Just about everyone in the wasteland knows to avoid Andale, and they're right to do it. People wander in here, and they don't wander back out." – (When wearing a Vault jumpsuit)