With new services coming from companies like Lego and Disney that are aimed right at them, it might be wise to keep some glass cleaner nearby.

These services are increasingly social in nature, places where your children can interact with other children, and they are becoming a central part of the business plans of the people who make TV programs, toys and cereal.

Your child might get a plush toy bearing a code for a free gift on Neopets, or a gift card for a free month on Club Penguin. How can parents sort out the best options among these services? One trick is to think about how they make money.

Because of multimedia Web software and faster Internet access, there are now lots of sites that mix habit-forming games with ads. One is even called Addicting Games (www.addictinggames.com), and it is run by Viacom’s Nickelodeon division, a company that has a clear interest in attracting the minds and mouse clicks of children.

As visitors play (and rate) games like Indestructo Tank or Drag Racer v3 on the site, they see ads for Intel computers or other Nick sites. Clicking through and hopping from site to site will give a child a crash course in the latest Bratz movies or Hannah Montana concert.

Another group of sites mixes free content with paid access, using a technique known in the industry as the velvet rope approach. Children are invited in to set up an account, and they become invested as they adopt pets or decorate homes. But then they are teased with features that cost money. Real money.

Later this month, BarbieGirls will be retooled in this way. Last year the site required the purchase of a Barbie MP3 player for access to certain content, an idea that has been abandoned. In the new version, children will be able to get in free and chat with others, dress up their on-screen dolls and decorate a room. But a collection of some games and fashion items will be off limits unless they become a V.I.P. player, which requires cash. VIPs are distinguished from the other Barbies by their sparkling tiaras.

For younger children of both genders, ZooKazoo (www.zookazoo.com), out this month, is similar to the popular Club Penguin but has an educational slant, with games that involve sorting objects and saving the environment.