You can edit the file at /etc/xdg/autostart/light-locker.desktop and add the line

Hidden=true

as documented here. This will disable light-locker globally, which is nice if your family starts complaining that they cannot unlock their desktops. You can copy the file to ~/.config/autostart first to disable it for a specific user only.

Source: 14.04 – How do I disable automatic screen locking in Xubuntu? – Ask Ubuntu