There are an estimated 200 virtual worlds for kids in the works. But SuperSecret thinks it has the secret sauce. And it’s just raised $10 million in a first round of funding led by Opus Capital.

The company has been building the world since late 2007. Kids log into the world, choose an avatar, and then wander around and chat with other players using a phone-like chat client that is a lot like an instant messenger client. They can play as many as 35 familiar casual Flash-based games like Bloons and Fancy Pants Adventures. But the “meta game” is about how the kids in the cartoon world can grow older and gain privileges.

You start at age 10 and can grow to age 18 by achieving things in the various games. You can get a couple of “age points” as well as “spenders” each time you play a game, for instance. Age points accelerate your aging. And you can use spenders to decorate your avatar or room.

With age comes privileges. At 15, for instance, you can get a bank account. At 16, you can drive around the virtual world in a car. At 18, you can vote. The idea is to give

10-year-olds, who aspire to be older than they are, a taste of what it’s like to be older.

Does it work? So far, the San Francisco company has had great success since its quiet beta launch in January. There are now 150,000 registered users, all from word of mouth. On average, they play 55 minutes a session.

SuperSecret cuts deals with third-party game publishers and developers to include their games on the site. That’s both a good thing and a bad thing. It’s bad because you can find those games on almost any site, often for free. But it’s good in that they’re games kids already like to play. Chief executiveTed Barnett says that it’s best to put familiar hits in front of kids rather than taking a risk on what could be boring games.

Barnett (right) said that by April, the company will launch the first of three distribution deals that will help grow the number of users. The initial part of the game is free until your character reaches age 13. After that, the company charges a $4.95 a month subscription. Barnett thinks the company can pull this off because kids are likely to be loyal to a site where they have an avatar and can come back for long-term rewards.

Since parents really hate surprise fees, the company is not going to use a virtual goods model where kids can buy things in the game for real money. It also will not show ads inside the world.

SuperSecret built the aforementioned chat client to be safe for kids. It won’t allow any inappropriate words, and it flags the user if someone inappropriately asks for personal information such as an address, phone number or email. With the chat client, you can see which friends are online and join them easily. As with social networks such as Facebook, you can send them funny messages, such as a Sneak Attack, where you lob tomatoes at them. In a way, this makes the site an introductory social network for kids.

While some puzzle games may be educational, the primary aim here is entertainment, Barnett said. As yet, there is no user-generated material, since that raises safety risks for kids (if someone created something inappropriate, it would take a while for SuperSecret to find out).

Barnett won’t say exactly how many employees he has, but the number is less than 50. The team includes veterans of Apple, Leapfrog and AOL (Barnett sold his previous company to AOL). His co-founder is Tony Espinoza, who serves as president and is a veteran of Apple, IBM and several startups. The company raised a $1.5 million round of seed funding and then, as part of the same round, raised another $8.5 million in the round led by Opus Capital.

The competition will be tough. Among the 200 virtual worlds out there are established giants such as Disney’s Club Penguin, Webkinz, Pirates of the Carribbean and Stardoll.