Earlier quoted context omitted.
Combine that with ensuring that your child's computer is in a public place, not their bedroom. Not really practical advice in an era of tablets and mobiles.
Very practical - don't give your <12yo child a tablet or smartphone. They're not necessary parts of human existence, and certainly not of a child's existence.
Most of the "harms" of screen time disappear when adjusted for things like socio-economic status: http://fivethirtyeight.com/features/screen-time-for-kids-is-...; http://daily.jstor.org/screentime-feminist-issue. Basically, like most parenting advice, exhortations to limit kids' access to screens is based on pseudo-science and old wives' tales. It should not be taken seriously.