The biggest threat my parents used to keep me in line was that they would take away my computer. If only they had Steam Family Options back then they could save themselves a lot of trouble (and back aches) and just lock me out my account. The feature, which up until today was in beta, is now available to all users. It allows you to limit an account's access to the Steam Store, Library, Community, and Friends with an additional PIN.
You can limit access by creating a separate account (for your kid, presumably) or just enable Family Options on your own account. If you've downloaded the latest Steam update you can enable Family Options by going to Settings, where you'll see the new Family tab.
With Family Options enabled you can curate the account's library of games, removing content inappropriate for children, like Left 4 Dead, Killing Floor, and Spelunky. No child should experience the abuse and humiliation that the latter will subject them to. I can barely handle it as an adult.
I just went through the process and it was very easy to enable, create a list, and disable. I was surprised that at the moment there are no time related options. You can't limit access to games at certain times, nor can you limit the amount of hours you can play a game during a day, week, or month.