First, we will take a look at Web of Trust.
The Firefox WOT add-on sits in the upper-left of the toolbar section.

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When you click on it, this comes up.

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There you can see the ratings of others (the color-coded circles) and rate the site yourself (the red-to-green-colored bars). This is WOT's technique. It uses the ratings of users to show whether they think the site is good or bad.
Not only do the results show when you visit a site, but also when you search.

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There is one other important method to see the ratings. You can set WOT to block access to the site or display a warning when you browse to it.

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Then when you go to a bad site, this is a warning you get.

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Or if you set it to be blocked.

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This can actually be a good free filter for your child, if they don't know how to change the settings.
There are some cons with WOT:
- A site might not be rated.
- A site may trick most people to believe that it is good, but it really isn't. So that affects the ratings.
- A site may be good, then becomes bad (malicious or "not-good-for-kid's-eyes" content added).
- A site may be bad, then becomes good (maybe a site was "hacked" for a bit, then the offending content was removed. But the site could suffer from the negative ratings).
In spite of these, I still recommend that Web of Trust be in your security set-up, but don't always rely on it.