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?