Good Co-Op or Multi Games for Younger Players?

How about the " Trine " series. It's a nice platformer with beautiful graphics and great gameplay.
Currently it has a 85% discount.

ibb & obb maybe? It was available in a bundle quite recently.

Warthunder, Warframe. Even though those are kind FPS'ish they dont demand much of fps skill like CS or similar. At least that how I see them.

How about Command and Conquer red alert 2? You can play skirmish togheter against eachother or in the same team against the computer. my friends kid plays it too. It is often for sale in a c&c pack for like 5 dollars. And its a very good game.

Rayman Legends
Trine 1
Trine 2

you can also try
Space Farmer, puzzle, and it's easier than Portal ;)
Risk of Rain, action platformer

Brothers - A Tale of Two Sons and Trine.

Lego series are really fun even if you are past their target audience age :D. Lego star wars, Lego Batman, etc.

How has no one said Castle Crashers?

Trine, castle crashers, battleblock theater, the expendabros, capsized, fighting games :3

http://store.steampowered.com/tag/it/Coop%20locale/#p=0&tab=NewReleases

Lego games

I will add one of my favourite games for co-op - Monaco.

EDIT: Oh, I forgot about the classic - original Contra for NES! Best game for manly co-op action.

Risk of Rain, Binding of Isaac Rebirth, L4D2(with split screen, search how to on internet), racing games,

Terraria, Castle Crashers, Battleblock Theater.

Thanks for the suggestions. I'll have to look into them more closely, but from the titles I recognize, there ought to be something in here that will work for us. More suggestions are certainly welcome though -- the more ideas I have, the better. :) Thanks!

What about Magicka? It goes on special every other weekend

Happy Wars its free and fun, i think can play all ages.

My young nieces liked many of the games already mentioned. They also like Worms Armageddon.

Counter Strike: Global off...erm I mean Rayman Legends? XD