[revolvingocelot]

Feh, it works on your machine, eh? :D

But seriously, you need Family Sharing. Make your kid an account, then set your account to Family Share with his. Now, he can play games from your library as long as you're not playing any, even if your account is online too.

So, when you want to play some singleplayer stuff, log into your account and buy and download the game as you normally would, whilst your son plays with his friends. Now kill Steam's internet connection on your computer, and play your freshly purchased cheap singleplayer game without bothering or having bothered your son.