Today, Valve announced that its Steam Family Sharing program is available for all users. This initiative allows Steam accounts to authorize devices to access their library on other accounts. The intention is for each family member to have their own account while being able to borrow games from one another. This can also extend to "their guests". It does not include titles which use third-party DRM, accounts, or subscriptions – Valve obviously does not have direct control over them.
There are other rules and restrictions, of course, but the account and device limits are quite high: 5 accounts across 10 devices. This does not get around region locks and a game which is VAC-banned cannot be shared. Ultimately, be careful sharing your games with your kids if they are jerks.
To setup Family Library Sharing in the Steam Client, go to View > Settings > Family and start to authorize and manage other computers. Just do not allow Cheating Charlie. For more information, check out Valve's promotional site and FAQ.