K9 Web Protection is a child's (or children's) web filtering and blocking software that is extremely configurable. It offers: web-page filtering, word block, site block, category block, and the Internet time feature. You can set what hours of the day you want the Internet on the computer to be blocked, and can customize each day of the week. All settings are accessed via the web browser, and are protected by a password, which you could give to someone else and not know yourself if you wanted to completely block access. It works cross-browsers; one configuration covers Internet Explorer, Firefox, and others all one the same machine.
As for managing it for each child, I would recommend setting up separate user accounts, and then configure K9 Web Protection in each account to how you want that child to access the internet...
A big point is that you can turn off different aspects of the program, like turn off the webpage filtering, and just use the time feature, or whatever...
Have used the program myself with great success, highly recommend! absolutely no crapware or trialware or anything, just plain free! all you have to do is give them your email for a product key, and it works...they don't send any spam, or bug you about anything...
K9 Web Protection: http://www1.k9webprotection.com/
Did I mention that it is FREE?
This question is relevant to me as well. I am looking for the simplest thing: limit amount of time, don't want any additional bells and whistles and DEFINITELY don't want any advertisements or sending of browsing history to 3d parties etc. The answers provided so far do not seem conclusive. – Gnudiff – 2014-06-28T08:03:00.283
Microsoft Family Safety has way to limit total screen time on multiple devices, but per day only – George Birbilis – 2016-11-14T15:00:31.080
Seems like it would be something pretty neat if there were a windows service that could monitor login time, and then auto-logoff when the time was reached - I wonder how hard that might be to write -I've wanted something like this at one point of time or another. – Spig – 2010-09-03T15:47:44.323
5Doesn't sound so hard for one to implement: Write "time,duration" to a file in a hidden folder with the right permissions: C:\Times\[UserNameHere] and update that file each minute, when the duration goes over a limit then logout, when the time was from the previous day then reset. Now you could create a scheduled task for each child... Warn on limit-1, etc... – Tamara Wijsman – 2010-09-04T17:37:43.893