There are tools for dealing with the problem, most commonly, filtering software that attempts to bar sexual, violent and other objectionable material. But these can frustrate kids and parents, by either blocking too many things or not blocking enough.

Avatars help guide users

Some other approaches, such as the parental controls built right into the latest Windows (MSFT) and Macintosh (AAPL) operating systems, offer parents more control by allowing them to specify what Web sites a child can access. But that requires close and constant involvement by the parent as the child seeks access to more Web sites.

This week marks the launch of a parental-control service with a somewhat different approach. It’s called KidZui, and it aims to offer kids a safe subset of the Internet where they can roam freely without triggering parental worry. KidZui, for children ages 3 to 12, hopes to emphasize the positive, rather than the negative.

The service, from a San Diego company of the same name, claims to encompass 500,000 safe sites, photos and videos, ranging from pop culture to science, comics and games to history. You can watch the latest “American Idol” contestant, learn about dinosaurs, delve into history or visit popular kids’ sites, such as Webkinz and Club Penguin.

The sites, photos and videos included in KidZui are approved by a team of about 200 parents and teachers across the country, and are ranked by age, so that a site that might be right for an 11-year-old isn’t served up to a 4-year-old.

While a child can establish a list of friends in KidZui, and can share content with them, there is no instant-messaging or email function.

KidZui isn’t free, and it can’t be accessed via a regular Web browser. Instead, you must download a special KidZui browser, from kidzui.com, that runs on either Windows or Macintosh computers. I tested it on both platforms, and it downloaded quickly and installed smoothly.

The service nominally costs $99.95 a year, or $9.95 a month, but there is a 30-day free trial and an introductory rate of $49.95 a year, or $4.95 a month. It has no ads, other than those already present on Web sites kids visit.

A key selling point of the service is that busy parents can simply set up KidZui and trust that their kids will be safe online. To that end, the program can be optionally configured, so that a child can’t escape from it to use the computer’s standard browser, for example. A parent can set KidZui to launch when the computer starts up, in full-screen mode. In this mode, KidZui automatically disables or hides the common keystrokes, icons, commands and techniques that allow users to switch to, or to start up, other programs.

In addition, when KidZui is running in this locked-down mode, the child can be barred from quitting KidZui without a parent’s password. In my tests over the past week, I found some loopholes in this lockdown system, but the company plugged each leak I turned up. I can’t swear that a clever kid won’t be able to escape from KidZui, but the program blocks most obvious exits.

Inside the software, the company has tried to create a fun, lighthearted world. Each child is represented by a “Zui,” a cartoon-like character that can be customized with hair, clothing and other features. There are lots of sound effects, and kids can rate content with illustrated tags ranging from “best” and “cool” to “boring” or “gross.”

When a child types in a term like “ocean” KidZui offers a list of related terms as well, to guide further exploration. If a child types in a search term or a Web address that has been banned from the KidZui universe, a message appears saying “This page isn’t available on KidZui, but your parents can add it for you.” This applies not only to terms typed into KidZui’s own search bar, but also to terms a child enters at sites like Wikipedia or in the search boxes embedded in other sites. The main pages of Google and Yahoo can’t be summoned.

If a search or Web address is new to KidZui, a different message appears promising that it will be reviewed.

I did find some holes in this system. For instance, I was able to get to The Wall Street Journal’s Web site and do an internal search on “Spitzer,” which turned up a story on the former New York governor’s sex scandal.

Parents can get detailed reports about the KidZui activities of each of their children and can tweak the content they can see by adding specific types of material, such as “athletic violence,” and approving or blocking specific Web sites.

For parents who want to allow limited Web use by their young children without constantly micromanaging their online activities, KidZui may be worth a try, but don’t expect it to be perfect.

Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

Of course, in many families, these problems are solved with good parenting skills, and establishing trust and limits. But even strong parents could use some technological help.

For years, add-on programs have attempted to give parents some control over what children can do on the computer. Some of these have been OK, but many have had weaknesses that were exploited by kids, who are typically technically savvier than adults.

Many parents, however, don’t realize that the latest versions of the two main computer-operating systems, Microsoft’s Windows Vista and Apple’s Mac OS X Tiger, have parental controls built right in.

On both platforms, you can control even which programs a child can run. This is key, because it prevents kids from running alternative Web browsers or other programs that may not be susceptible to parental controls. Both also allow you to specify which Web sites a child can visit, another crucial feature.

These built-in controls are free of charge and fairly easy to use. Even better, because they are designed by the same companies that built the operating system and aren’t bolted on afterward, they can impose limits in ways that kids may find harder to evade.

I have been testing these built-in parental controls. While they aren’t perfect, I can recommend them as powerful tools to help parents get a handle on their children’s computing and online activities.

On both Windows and Mac, the trick is to make sure the computer used by a child has multiple accounts, or logins. One, for a parent, should be set up as an “administrator” account, the type that grants its user powers to change various settings, including the power to establish parental controls on other accounts. This administrator account should be protected by a password — and this password should never be shared with the child. If the child knows it, he or she can log in as the administrator and weaken or remove the controls.

In addition, you should set up a standard, or more limited, account for each child who uses the machine. People logged in via these accounts can’t change many settings on the computer and can’t override the controls.

Once logged in to your administrator account, you can apply different limits to each child’s account. In Windows Vista, you can find the parental-controls settings in the Control Panel, under the heading User Accounts and Family Safety. You must be using the Home Basic, Home Premium, or Ultimate versions of Vista to apply these boundaries. They aren’t available in the Business or Enterprise versions.

On the Mac, in Tiger, you can find parental controls in the System Preferences program, by clicking on the Accounts icon. You select the standard account you want to control and then click the tab labeled Parental Controls.

In addition to restricting which Web sites kids can visit and which programs they can run, Vista, but not the Mac, includes an extensive system of controls on games. On the other hand, the Mac allows you to block a child from using a printer or burning CDs.

The Mac system, but not Vista, allows you to specify exactly with whom a child can exchange emails or instant messages — as long as the child is using Apple’s own built-in Mail email program and iChat instant-messaging program. To ensure compliance, you would have to limit the child to using only these programs and not competitors or Web sites that perform these functions.

In Vista, you may be able to set up similar limitations within the individual email or instant-messaging program, but it’s easier on the Mac. If a nonapproved person attempts to send your child an email, the Mac system can even forward the email to you.

On the other hand, Vista, but not the Mac, allows you to set time limits for a child’s use of the computer. Another strong Vista feature absent on the Mac is a detailed report on the child’s activities that can be emailed to the parent — though the report doesn’t include email and instant-messaging activities. Vista also can filter out Web pages based on content categories, such as sex or drugs.

Apple is planning to add a time-limit feature, Web-content filtering and a new activity-logging feature in its forthcoming Leopard operating system, due in October.

Determined kids wanting to view pornography or contact strangers could probably find ways to evade both systems. It’s awfully hard to shut down access to everything of which you disapprove without also blocking access to valuable content and functions. But the logging and reporting features should at least make evasions detectable after the fact.

So, if you want to put some technological muscle behind your parenting, don’t overlook the parental-control features hiding in your own PC.

• Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.