Good free/cheap games for kids? + more Thread

Help is greatly appreciated here, I'm looking for games that would be suitable for kids aged from 6.
Nothing too demanding if hardly at all, within reason as the PC they'll end up on is fairly old, though I'll be updgrading it as best I can, but it's going to be far from a powerhouse, probably at best half life 2's system requirements once I get a gfx card in it and new CPU.
Looking to get a friends girls into PC gaming, or at least games beyond those prom makeovers and spas on a tablet. Generally anything goes even mild horror (I'm going to show them Slender at the moment), except the obvious adult games with strong language or violence.

Only other requirement is that they don't use online only DRM/activation, like Steam or Origin as there's no internet access at the moment.
The more the better, a nice big library of games would keep them occupied.
Any games which only need the mouse to play or few keys is great, one girl has limited control of her hands/arms due to cerebral palsy so a nice simple to control game would work great.

Id say slender would mess up any 6 year olds brain, try some creative games and challenge them to use their creativity.

Ive heard of minecraft being used in schools.
Keep them off the online mode and you have a real time killer that they can standback and admire their creations, perhapse take some creative inspiration to create their own games.

Better to game for education, rather than entertainment at that age id imagine.