I recommend using the LW filter to block all video formats. You'll also want to block certain words, such as 'orgasm'. And, of course, you'll want to use it a bit, trying out the searches that your kids are likely to use.
More help here:
http://www.gnutellaforums.com/open-d...uld-aware.html
I don't know of any free download sites or networks that are kid safe. Still, it might be worth investigating. Perhaps others will add more suggestions here.
This is what you get for being a good parent.