Other popular tween online hangouts include Millsberry, a General Mills Inc. site that promotes good eating but features product placements for its cereals, and Numedeon Inc.'s Whyville, where tweens play games and earn clams.

Although these social-networking precursors for tweens tend to incorporate more safety measures than MySpace, Facebook and other sites geared toward teenagers and adults, experts warn that parents can't simply sign their kids on and leave them there, especially during the summer months when kids have more time to spend online.

"We want them to develop and grow physically, spiritually and emotionally," Meszaros said. "If they are on the computer three or four hours a day, that's time they could be doing other things. Parents need to be monitoring."

Step one is to decide whether kids should be there at all.

Jane Healy, author of "Failure to Connect: How Computers Affect Our Children's Minds — for Better and Worse," said kids may feel they are "going to be a hopeless social failure" if they can't participate.

Advocates say the controlled environment can teach kids important lessons about typing, communicating, caring for pets and budgeting — they must learn to work and save for the trampoline they want for their virtual room.

But Healy said these sites also teach kids to be "a good consuming member of the consuming culture (and) to need stuff to be considered successful or good."

She urges caution in opening the door to "powerful forces out there trying to intrude into your family life and personal relations with your child." Not only do these sites introduce commercialism, she said, but they also can take kids away from offline environments where they can learn to pick up body language and facial expressions.

Software tools are available to help parents control Internet activities, including use of these sites. Monitoring software can record a kid's chat conversations and whereabouts — secretly if the parent wishes. Other tools, some available for free, aim to block porn or limit when or how long a child can be online.

Parents should at least keep computers in an open room and surf the Web side-by-side with their kids now and then. A discussion on time limits is important because rules are far easier to impose from the beginning, and Club Penguin will soon introduce a feature for parents to set such limits on the site.

"As soon as the egg timer comes up, we're going to have a list of activities they can do outside," Merrifield said.

Parents should also start addressing safety and online etiquette.

"They can't be there every time they go online .... so it's even more important to spend more time up front teaching them how to be safe and smart," said Susan Sachs, chief operating officer with the nonprofit Common Sense Media.

It helps that many parents are now using the Internet not just for work but also for recreation, information sharing and other social interaction.

"When kids start to use technology, (parents) can be much more part of the process, as opposed to, 'Gee, this is all new and strange to me. I don't want you using it,'" said Peter Grunwald, a researcher who specializes in kids and technology.

Nonetheless, Grunwald said, "kids are using online services at an earlier age, and that means parents do have to exercise their role as parents and be mindful of it at an earlier age than, say, seven, eight or nine years ago."