The YouTube video, if restricted, is still blocked even when it shows up in ask.com search results (when YouTube is in restricted mode). You should block search engines that don’t support safe search. On the pcWRT router, you should block all search engines except:
When you download a YouTube video through a third party web site, the request goes to the third party web site, not YouTube. So you should block such sites. Of course it’s impossible to block all such web sites. But if search engines are blocked and you can’t get a direct link, it is harder to download “bad” videos from the download sites.