Well there's a lot of possibilities you know, but one main one is kids getting exposed to explicit adult material. I mean, sure it's okay to explore the net and all, but I don't think many parents want their kids on sites that they aren't supposed to be on.
Another would be cyber bullying, as I see that happening many times these days. There's nothing we can do though, all we can do is tell them to be safe on the net. What else is possible?