There's a clue left by FNaF4 that everyone missed, which has only recently begun to resurface in deep theorizing circles. Hidden in plain sight, FNaF4 sneakily reveals the crying child's identity:
The crying child is called "W ASD".
What is the significance of this name? Is it an acronym? ("William Afton's son Dave" perhaps?) Few people have caught onto this clue, but even fewer have realized its implications for the larger story. FNaF4 is not the first we've seen of W ASD.
W ASD was not a new character introduced by FNaF4, it was the reveal of the identity of a character we'd already met. The spirit who haunts all 5 Classic animatronics, animating them in Follow Me, is the crying child.
But it goes further still. W ASD has been a recurring character as early as FNaF2.
According to TakeCake, W ASD was already haunting Freddy at the time of Charlie's murder. This resolves the death order debate, clearly placing the bite as the first death of the original story.
Apparently, W ASD also haunted the Puppet during Follow Me. Could this explain the Puppet's ability to give the children life?
W ASD is also present for the MCI, presumably haunting Foxy, and for the FNaF2 murders, haunting Freddy.
In the games following FNaF4, W ASD is named during Elizabeth's death, the opening of FFPS, and the Security Puppet and Fruity Maze minigames. But now his name is only in the background, no longer directly tied to the characters.
What is the purpose of all this?
It seems that, all across the story, W ASD is present; even when not haunting the relevant animatronics, he still somehow lingers in the background. W ASD was the original victim, from the original location. After the bite, he haunts the Classics and Puppet, following along for all the central story beats. W ASD acts as story's initial mover and continual omnipresent force of nature, that first unique spark which would grow into the flame of everything to come, thus haunting the story as a whole.
This is the narrative point of FNaF4, always hidden in plain sight, appearing to all but recognized only by those special few who know how to look... Those who know the name of W ASD.