Callaway has the right idea. Don't wait to teach your young ones how to throw it properly. Just don't let them throw it in a game where they may throw it too hard or improperly. Curveballs are to young pitchers what *** is to most young kids. It is exciting and mysterious and if they don't learn about it from you, they will get it from their friends.
ONE TRICK *(NOT FOR EVERYONE)
The real question is how to teach it because you can't have pre-teen kids yanking down real hard on a curveball. I teach a classic 12-to-6 that begins way over the top instead of release (basically, the wrong way, really way to soon). You end up with a big loopy "curve" that is really a change-up. Why you say! Again, kids are kids and they want in on the secret. This way, you have taught them a "curveball" that doesn't put much stress at all on their elbow or shoulder. NOTE: make sure they have good follow-through to protect the elbow.
Finally, kids have been throwing curves for over a century without arm injuries. The reason I believe for the recent increase in arm injuries is because now kids are throwing curves in games to get outs. Their throwing them hard and often. If you watched the Little League World Series last summer, it was the number 1 pitch for a lot of kids. (remember when pre-teen kids pitched just to get the ball in play....those days are gone.)