Internet monitoring software

[Floop70]

Does anyone have any suggestions on software that will allow me to limit the time any particular user spends on the internet? I've got 4 kids and would like to give them each 30 or 40 minutes of time on the 'net each day without having to watch and make sure they don't use an hour or something. While monitoring what sites they go to would be nice, it really isn't my main goal. Our computer sits in the living room so I'm not worried about them visiting inappropriate sites (this hasn't happened in the last year yet anyway)

I'm thinking something that will force a login for internet access and then cut off access after a specified amount of time. Any suggestions are appreciated

Thanks,
Bruce