The Sims is a series of virtual life games that allow players to control human characters and guide them through their lives from infancy to old age and death. The later games in this series, The Sims 3 and The Sims Medieval, include more role-playing elements like quests and story-driven scenarios, but they lack the fantasy element. They also play more as simulations than as role-playing games and lack the built-in social element that's so popular in Free Realms.
Second Life is like The Sims in that it focuses on humans in the real world, or at least a virtual appropriation of reality. The main difference between The Sims and Second Life is that the latter game is multiplayer and allows players to communicate with each other and collaborate in developing their characters. Given Second Life's more adult themes and player base however, this game isn't appropriate for a younger audience.
Finally, Runescape is perhaps the closest to Free Realms in terms of overall style. Set in a medieval fantasy world, this game is a more typical role-playing game with its focus on skills, combat and levelling. Still, Runescape offers a uniquely different gaming experience and misses some of Free Realms' marks.
It's important to note that to stay competitive, simulation and role-playing games must stand apart from games of shared or similar genres. The next time you're looking something new to play, try to identify the specific things you're looking for in a game. It may help you develop a clearer sense of what you really want to play.