rocket - No, I don't think you said they'd fall in love with the curveball. But I think they will...at least thats my observation over 14 years of coaching kids. If a 9/10/11/12 year old can control it (i.e. get it near the strike zone), then he can get easy outs with it. That makes him fall in love with it, his parents usually, and his coach usually. And then it gets used too much. Its also a reason I didn't practice it with my sons when they were young. I didn't really want them to find out they could have a lot of success with it. Plenty of time for that later.
On the major leaguer question...I just have no idea. What I know is what I said before...the kids in my area who threw it a lot at that age (or even threw it much at all) didn't enjoy much pitching success in HS. Like I said, I can only think of one, maybe two out of dozens.