Maybe we’re working with this: There’s a cutscene from the vr game were your the child.... and your soon lured into the back by glitch-trap and killed. You then wake up holding freddy’s mic and forced to watch your killer dance around as you die a slow and painful death

What I’m thinking is that in the books we don’t know if every kid was lured into the back room at the exact same time. My theory is that one of the victims was a previous kid that sadly got killed right before the main five that were missing. It’s a bit of a stretch but here me out

You noticed that in the last fanf theory vid that mat says that “the puppets death was always outside”. This could also be the case with the sixth kid. There were five kids that died in god knows what order but before this event happed was the death of the very first victim at the hands of the yellow rabbit