Gnu/Linux games are not quite up to the gfx quality of Windows/Mac etc games. I have tried some of the FPS games and they are good fun gfx on a par with HL and maybe even better.
There are tons of good games for the younger person and I have tried and can recommend as good fun,
Tux Typing
Frozen Bubble
Super Tux
Super TuxKart
Extreme Tux Racer
Trigger
Scorched 3D
Free Col
Battle For Wesnoth
Free Droid RPG
I have tried to play,
Yo Frankie - gave up trying to install as I had problems with the Blender version needed for my version of Arch. I really want to play this and will try again in the autumn.
Danger From The Deep - installed ok on Mandriva(I think) looked good but too many keyboard keys etc etc for me but it looked pretty neat.
Have not played but will investigate,
Spring 1944 - This looks really interesting and is supposed to be very acurate for the equipment and maps. You can play as the Germans aswell.Another for the autumn or deep winter.
FreeDink - The game features the twisted adventure of young Dink Smallwood, a pig farmer who leaves his town and goes on a quest to save the world. It is the title that gets me, mebees I'm just a country bumpkin at heart.