Parent to child no matter how old works for me,as long as it's not overused, but no way do I see Dean as Sam's substitute parent - far too much of that goes on in fan fiction for my liking and that's what the "kiddo" phrase implies to me.

I just don't see Sam standing for it. He balked and fought Sammy for quite a while before he gave up - I really, really doubt he'd let Dean get away with "kiddo."