Parental controls not logging off user
Looks like you are doing everything right. Auto logout will only work for the user that is currently logged in.

The easiest way is to reboot the system. Either automatically at night or if you need to get the system under control quickly. 

I found this discussion on the apple support boards (https://discussions.apple.com/thread/1402679?start=0&amp;tstart=0) that advocates using Activity Monitor to kill the process.

Or go hardcore and write a script that will occasionally kill all processes of the kids users:

killall -u username