Who can contact my kid in an online world?
In a closed virtual world such as Club Penguin, your kid can interact with any other player who is on the same server. (Sites separate players onto different servers so as not to clog up the works.) There are different levels of interaction for kids who have subscriptions versus those who are using the website for free.
Your kids may or may not know the identities of the other players with whom they're interacting. For example, friends may tell friends which server they'll be on and at what time they'll be on and what their online names are. Kids can log onto the site, choose the appointed server, and look around for their friends. Along the way, though, they will "meet" and interact with lots of other players whose identities they do not know in the real world. On sites for young kids, the types of interactions are limited. Many have "canned chat," which means they can only select from a list of phrases to communicate with others.