It's been a while, so I'm a little sketchy on the details, but Dean is just a child (5-ish) when he meets Cas - I want to say Cas is his teacher, but maybe not. Dean is drawn to Cas right away and is super possessive, getting all growly when Cas shows interest in anyone else, even though he doesn't understand why yet, Cas knows right away that Dean is his mate, but puts it in his pocket for the moment because Dean is a kid. Mary and John both try to get to know him but John is all twitchy about a grown man perving on his son even though Cas refuses to make a move (no matter how much Dean wants him to as Dean goes through adolescence) until Dean's old enough.

Anybody know what I'm talking about? I'll be your best friend :)