That depends on what you watch. The most damaging thing on
YouTube is usually the comments section. Using a script-blocking
plug-in (like NoScript for Firefox) and a malicious page blocking
service (like WOT for Firefox) makes browsing safer in general
though.
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Namo CyberFence allows you to check and find out what videos
your kid watch. It also blocks adult video clips from YouTube.
Otherwise, you can block whole YouTube.com
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oh my freaking god, please tell me your kidding
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Make sure you have the latest version of Adobe Flash Player
installed on your computer. You should also make sure that your
browser or any website filter is not blocking YouTube from being
visited.