There's a great solution to this, coming from the Windows XP world (sort of)...
When my ten-year-olds are playing a game on their mother's computer, they know that if they pause it and walk away for a few minutes the screen-saver will kick in and Mom will, more likely than not, -not- drop what she's doing to come and enter the password yet again for them. So when they need to step away from the computer for whatever reason, they issue the following command:
Peter, move the mouse around until we get back.
... and, if he's feeling cooperative, their four-year-old brother will keep the timeout from expiring. This is approximately as reliable as everything else that happens on a Windows system.