You know I'm suddenly reminded something like a year or so back, where it was put out that a father and his son were playing GTA together. He was ignoring the game missions and instead was following the traffic laws, and asking what color cars were, and teaching the rules of the road rather than going full violence... So if you're co-op playing and steering in the right direction, a lot of other games could be an option.

Brutal DOOM 64.

Kidding aside, Contraption Maker is worth a shot. VGA versions of the King's Quest games would also work - you can get KQ I through III as freeware through AGDI. Rayman Origins should be worth the time - whenever the title character bites the dust, the controller can be passed between you and the kid.

TIM would be a fine thought for a child. Bit of an antique these days.

The Lego games are perfect since you can play them together with your grandson. As far as I know all of them support two players on one computer.

Other great choices that weren't mentioned yet:

*) Dust: An Elysian Tail
*) Ori and the Blind Forest

Anything from Humongous Entertainment.

On GOG? Pajama Sam!

On Steam? Like zeogold said, check out Humongous Entertainment, mainly (besides Pajama Sam) Freddi Fish, Spy Fox, and Putt-Putt.

I would have gotten a hooker and taught him the basics of haggling.

Maybe <i>The Manhole</i>?

Hmmm... I'm not sure I'd recommend those. Great games, but reactionary and heavy platforming suggests they probably won't be able to do well with them.

Costume Quest and Terraria ^_^

Try Psychonauts.

Not really sure that's suitable for 6 year olds.

I'm guessing it would be a better game for 13+.

Maybe, but when I was 6-8 I play in various quest games and fps like doom. :D Very childish games was boring for me. Except Timon & Pumbaa's Jungle Games, but this game not available on gog and steam.

Botanicula.

Played it with my two boys at this exact age and it was a smashing success.

Gomo is good too, point and click adventure without words. Puzzles are not very difficult at all.