Wine and Linux for the client machines should work well for your needs. Ive seen that most games work quite well under Linux+Wine. If Wine isnt enough, possibly CrossOver Games would be better.
Windows prior to Vista doesnt really offer capabilities that you are requesting, as far as I know. And Windows is still expensive...
But on the Linux side, there are a few tools that can do that.
For GNOME, there is GNOME Nanny. Though it seems restricted to time session management and web activities.
A more universal set of tools is available too:
-
TimeKpr- Controls login and session times using PAM
-
WebContentControl- Despite its name, it does more than control website access. It also includes scripts for controlling application access.
-
A new *in-development* GUI called GChildCare is being made to succeed WebContentControl.
Im not sure if there are other tools out there that work. I think Mandriva Linux actually provides integrated support for parental controls similar to GNOME Nanny and TimeKpr. The scripts from WebContentControl would let you block which applications they can run, though that isnt that big of a problem on Linux if they dont have a compiler or root access.
KDE itself has a configurable Kiosk mode, which can help control KDE itself.