Good "Family Friendly" Coop Games from Steam

Just gotaaaa 
 
 Hey all. 
Got a average equipped PC for the family for xmas (has a nvidia ION graphics chip). 
Tried running Trine on it and it runs ok. 
 
I'm wanting to get some coop games now that there are finally two steam capable machines in the house. However my younger brothers can't play anything excessively violent. What does the GM community recommend?

Coming from a man who loves co-op games. I can help as much as I can.
 
There are not that many co-op games on Steam actually, and all of them are pretty violent.
Killing Floor, Modern Warfare 2, Kane and Lynch: Dead Men, Left 4 Dead, Left 4 Dead 2 (can turn gore off, and it's just as good as a T game), Rogue Trooper (The multiplayer is actually a co-op mode), Titan Quest (Like Diablo, but set in Greece, rated T and blood can be turned off), Dawn of War II (Has co-op campaign, Rated T I think), Neverwinter Nights 2(Actually this isn't a steam game, but it's on Direct 2 Drive).
 
You can look into those, and judge the level of violence yourself. Kane and Lynch is the most mature out of all of those.

How violent is too violent?
 
I mean, I can suggest plenty of great co-op games, but most (maybe all) of them revolve around killing a whole shit load of dudes.

" @Jimbo:  Titan quest looks like it's probably ok so around that level. "

Titan Quest was a pretty great suggestion tbh.
 
Soldiers:  Heroes of World War 2 is fantastic - one of the best co-op games ever imo - and mega cheap, but it can be reasonably tough.
 
Bionic Commando: Rearmed is supposed to be awesome but I haven't played it.

There's that Republic Heroes game.. I can't imagine that being TOO bad.

Trine is good, but local coop only.

How old are they?

11 and 12. 
Local coop is fine.

RTS games might be a good idea. However, the implications of the vast majority might infer something you don't want to. Dawn of War 2 is a good game, and you could have a great time playing that, teaching team management and such, and the bright goofy characters could be entertaining, albeit a bit gory for children.
 
I'm sure there's other stuff. 
 
Something I did a while ago with a younger family member was play an adventure game through with them. Something like Monkey Island/Indiana Jones/Day of the Tentacle, they really bring people together as you try to progress with people. Not co-op at all, but a wholly fun family experience!

hey you, buy a Wii

Probably someone modified Torchlight for co-op already.