There might be an option in Parental Controls. You would need to create a non-administrative account.
(I'm curious myself, so I'm going to do some research on what can be done through Parental Controls.)
Edit: my guess it that the only way you might be able restrict shutdown is by using Simple Finder. I haven't tried it myself, so I don't know which menu items are removed, though most are.
Of course, there's still nothing to prevent them from pressing the power button. That will bring up a restart/shutdown dialog (unless they hold down the power button long enough to force a power down).