Honestly at least one of the kids would be really relieved to know they aren't the only ones like this. Probably Frisk. Their world isn't an abberration!

Feeling of being watched. Chara's skin crawls something fierce, and they start scratching at their arms up and down through their sweater.

They can believe him, much as they hate the thought of it - it explains in hindsight his knowledge of so many other selves.

"You were watching," they say, the tone flat and clipped. "The entire time."

This is hitting certain buttons hard. One of their hands unconsciously scratches at the back of their neck.