I mentioned a while ago that I setup our kids' iMac with Parental Controls, so they wouldn't be able to modify the state of the computer. For example, I did not give them access to modify the Dock, so their programs will remain easy to access. I also did not give them access to "Open all System Preferences", so they couldn't change the system settings.
I started to notice something strange. Some programs were starting automatically, that I had not set up. This was strange to me as I thought that the only way to add login items was through the Accounts Preference Pane, which had been disabled.
I then discovered what was happening. When our daughter uses the mouse, she tends to click the right mouse button. When she clicks on an icon in the Dock a menu appears, with "Open at Login" as one of the options. This is how she was adding items to the login items.
This option should be removed from the Dock menu if opening all System Preferences have been disabled.