I recently joined a group of privacy experts working with a Spanish foundation dedicated to children's issues to think about how to help protect kids' privacy online, in particular in social networking services. We've just had one inaugural meeting, a brainstorming session. It's too early to say which approach the group will take. But for my part, I recommended a crowd-sourcing approach, where we encourage (sponsor?) an open-ended contest to invite people to create videos on YouTube where kids talk to other kids about privacy. I doubt a top-down approach would work, where governments or corporations lecture kids about what they should or should not do online. I think kids will react more to videos by other kids, who talk about sharing with their friends, what happens if they share personal stuff with the wrong people, how to make good choices.
If you have a better idea about how to approach the challenge of sensitizing kids about the privacy risks when they post stuff online, please let me know, and I'll take it to the group.