Ha! It have something to do with runlevel. I make computer runlevel 4 with /etc/inittab so it boot straight to KDM. Then I add parents' to groups 'power' and 'plugdev' so now they can use comp and CD's will automount. And they not know root password so computer not get messed up.
I learn how to do it here:
But then sound not work. So I try runlevel 3 again and sound work. I think maybe add parents' to another group and sound work again in runlevel 4. I not know how to fix this yet but maybe I can figure out.