MMOs.com has a list of family-friendly online virtual world games, including "Club Penguin" by Disney Canada and "Lego Worlds" by Warner Bros, which are aimed at younger children. "RuneScape," "Maplestory" and "Realm of the Mad God" are more appropriate for teens and older children.
"Club Penguin" casts the player as a penguin avatar in a winter world and is specifically aimed at children in the 6-14 age bracket. "RuneScape" is an open fantasy world where players train skills, such as woodcutting and fishing. It includes player-versus-player elements and is meant for children aged 13 and above.
You can find suitable online games by checking the minimum recommended age ratings of games on review websites, such as CommonSenseMedia.org. Features to protect children such as profanity filters and parent controls are usually noted on official game websites.