That is too bad, because I consider this one of the safest sites out there. But, as the parent of an almost 10 year old, who is becoming addicted to the computer, I can relate. Everything in moderation is fine, and if I could keep her to 30 minutes a day with no argument I would have no problem with her computer usage. She is a very busy girl so I think any more than that would interfere with more important things like schoolwork. We also monitor what she is looking at too.
Have you tried talking to them about a per day time limit? And of course encourage them to look at this site. But if they agree, you have to agree to stick to it too. That is my problem with DD. She agrees, but then begs for just a little more time. That signals addiction for sure.