It’s an interesting theory but I’m kinda hoping it’s not the Easily distracted by turtles and dogs shirt, – besides which Leaf’s quote refers to the First Men wiping out the Children, not the White Walkers, – so if this theory is true that would mean Bran was the one leading the First Men? So bran travels in time to the body of another man. Why would the Night king look like Brann does now? Also, the children do not say from you referring to Bran if they create the night King to defend themselves why are they needing to defend themselves from him, he was created to counter the threat they actually say from you men meaning they were at war with humans and they create the Night King.
