Ask HN: How do you monitor your young kids' computer usage?

As parents we can't be there at all hours of every day. Meanwhile, kids are being introduced to computers earlier and earlier. At some point, they will obtain a computer - maybe you feel it's the right time, or perhaps the grandparents give one as a present unannounced. Once you finally give in and allow them regularly access a computer of their own, how do you - a technical expert - monitor or manage their usage to make sure they don't get accidentally introduced to the darkest parts of the internet (e.g., bad images/videos, conversations with strangers, etc.)?