I play Magic occasionally and Yu-Gi-Oh! every now and then when I get time to while my girlfriend watches or joins me in Catan and Munchkin.
Who cares, right?

Does Greenman always sell Painkiller Black for $3 and change? Is there a cost to membership?

No membership cost for Greenman Gaming. However, prices can apparently vary and are only ever guaranteed for a 14 minute (IIRC) window during checkout. At all other times they are open to vary either upwards or downwards.

This is due to the trading system apparently, but I don't think it's ever been explained in any real detail. The only games that should not be affected by that are the ones that use SteamWorks (as you cannot trade them in).

Do you have only PC or maybe some consoles too?

only PC

Max and the Magic Marker, and perhaps some LEGO titles.

thanks,any other recommendation ?
(the little rascal always want to play Gmod)

You know, I have played Heroes of Might and Magic 3 with the 4-years old sister of my girlfriend. I was just storytelling while builiding the town and winning the battles (because the kid is into 'horsies! horsies!" I've developed the brand new Champion tactic for the Casle :D)

Maybe try something extremely colorful like Puzzle Quest and also tell a story during the gameplay?

What I also remember - if you're child is a girl - try some online games that requires clothing a doll - http://www.ubieranki.eu/ - like those here (just click the screen from the game to get to the one of them).

He's a boy,and my elder sister's child .

Air Forte is cool with young children, if they have a second player to compete with. Also, try Cortex Command:

http://www.datarealms.com

All my younger cousins love it.

For sure you should try some LEGO titles! Now you have a plenty of choice - Indiana Jones, Batman, Star Wars, Harry Potter - everything has its blocks version :)

I was just posting to recommend that! You fast killer ;)

Hehe which one? CC or Air Forte?

Peggle
Plants vs. Zombies
Ice Age
Madagascar
GTA?

Some casual FPS?

CC. I was playing this when I was once extremely bored and I totally got into ;)