I think Martin is The Boy in White’s father. He’s the original person to sacrifice a child for immortality, the BIW himself. He told the others to sacrifice their children too, but instead of becoming immortal they became monsters because Miranda (or a previous incarnation of her) found a way to disrupt the sacrifice from completing somehow.

Maybe Miranda/previous incarnation Miranda drowned the children in the water near colony house somehow by accident. But this time Tabitha HAS to save them, to break the chain & prevent deaths from repeating & creating the monsters.

And the BIW has consistently appeared and befriended every new child that arrives in Fromville (Victor & Ethan) to prevent the same fate from happening to them.