Info-Mac

I am new to this forum and OSX. May I seek some recommendation on how to set up some particular timer for my nephews :

I would like to restrict to the kids that maximum say 2-3 hours daily spending time in playing computer. They are allowed to log in and log out freely so long as the accumulated time not exceed the pre-set time is ok.

Once the timer reached the pre-set time, the computer will be automatically log out and can not log in again until the next day.

Is there any software or settings in OSX can perform this function ?

I have set up 2 users for them already.

My OS is 10.3.9

Thanks in advanced

Parental Controls, built into Mac OS X, already does this I believe.*

*I don't have kids so have never needed to test it.

Thanks Chasm
I could not find this Parental control in 10.3. But I see there is one in Leopard. May be I need to upgrade that before I can use this great function.

Thanks very much of your advices

And just to verify this, I have attached a screenshot of these settings in Leopard. I actually didn't know this was there, which is pretty cool.

Thanks D.
This is really a good thing to me. May be sooner I need to upgrade to Leopard which should be better control for kids access.
Thanks all