I worked for a while on something like this, although it was more of "Linkedin for Children". It's stempremier.com. Our big idea was that there are tons of kids being recruited (sometimes for somewhat early ages) for college and professional sports. However, there aren't a huge number of kids being recruited who aren't talented athletes. An example is a kid who doesn't necessarily have stellar grades, but loves the outdoor and possibly has an interest in electronics. These students are very sought after by telecom companies as linemen, but it is exceptionally difficult to make the connection.
One of the big obstacles we ran into was how to monetize. While I was there, our approach was to have schools pay for per user licenses for students. It was a hard sell.
I think there is potential if an app like this aims to help kids pursue a quality education and find what they are passionate about. I however, am skeptical about a kind of facebook just for kids that would likely seek to exploit the kids to sell their data to advertisers and would be at best a detractor from a child's education.