Google Image searching pretty much negates web filtering
If you search for some 'inappropriate adult sexual content' and use Google image search - you can view any image returned in full resolution (remotely pulled from the various porn sites), without Norton ever triggering the "pornography" category - as the requests are coming from "google.com".
You could argue you could force Safe Searching on (assuming they're using a compatible browser, which they won't be in our house hold as we don't support insecure browsers, such as Internet Explorer), but if you chose not to enable that feature filtering out content via category is moot.
Images, and adult images, are probably all that Google Image Searching might be used for - but the fact the ability to filter "pornography" is dependent on safe searching being enabled, should be a little less transparent to the parent. They may not realize how ineffective their settings really are.