NOTICE
It did a slight modification to get this addon running in Kodi Leia 18, but I do not maintain it actively. If you are interested in taking it over then please contact me via the Issues section.
All credits go to He-who-must-not-be-named aka Robwebset ;-)
PinSentry is an addon that will allow you to set a pin code that you can then force to be input when you play a particular video file or navigate into a given TV Show or Plugin. There is also an option to force the user to enter a pin when the system starts, and then restrict the amount of time the user is allowed each day.
Please read the wiki for more details: