Okay, first of all, did they decide to tell Jimmy or was it an accident?
An accident makes sense, because they were just joking between themselves about Superman.
But they didn't look too worried when he heard them, either.
I really wish Jimmy wouldn't be told in so many fics. I understand he's a close friend they can trust. (Obviously they wouldn't want to tell someone they COULDN'T trust.) And in this fic, in some ways they're closer to him because of Lois's half brother and the fact that they're closer in age.
But to me, whether they tell someone depends on whether they NEED to know, not whether it would "be nice if that person knew."
Lois always needs to know, in canon, in this fic, whatever - at least once she becomes romantically involved with Clark. (Or once she married him, in the case of "On the Other Hand" and "Learning to Love."
Bernard(or in this case, Sam), I can see needing to know because of the doctor connection for the kids.
But Jimmy doesn't REALLY need to know. It makes him feel like a close friend, and that's nice. But it's also dangerous. I always remember what happened in CANON when they told Lois's father. (That was "FAMILY HOUR", so it's probably an episode most would like to forget, but I think it's a good case for this arguement.) Even though they trusted him, he got hit with truth serum and was atttacked by the villian.
So it puts him in danger just because "he's a close friend." That's not the way to treat your friends.
Of course, they haven't met Trask or Lex yet, so maybe they haven't thought about the dangers. And they haven't heard about truth serum, either.
