The Internet has a lot of content that you may not wish to stumble on when giving children access to it whether at home or in a public access computer scenario (Library, community group, Internet cafe etc).
Putting safeguards on a home computer can sometimes be beyond the knowledge of many parents who may prefer to have a specialist put user specific Internet filtering in place (as many Schools and businesses do) whilst some operating systems allow the set-up of child-friendly accounts that help put some safeguards in place to ease the worries of many parents or guardians.
Glubble is a free add on for the popular Firefox browser and its derivatives (Flock and Seamonkey primarily) that puts puts a child friendly set of rules and interfaces in place but allows the asking of permission from nominated family or guardian members for sites and activities that may be blocked by the default access rules thus making it easy to individually customise.
A family noticeboard is available which lends the project a more human interface. Accounts can be set-up and tailored to each individual and changes can only be made via a password protected ‘responsible adult’ administrator account (provided the ‘responsible adult’ is not careless and shares the password).
Of course this kind of adult intervention would need to be used in association with the operating system user accounts configuration to perhaps stop the more technically knowledgeable children from just downloading an alternative browser in order to circumvent the well meaning protection put in place here.