From LedHed's Wiki
Overview
MythTV supports Parental Controls (Levels 1-4).
MythTV only supports Parental Controls at the file level.
After some googling I found this thread: http://www.mythtv.org/pipermail/mythtv-users/2012-December/344698.html
This thread suggests the use of MySQL Triggers to set the Parental Controls on files that reside in a particular folder. This approach works well because it doesn't require someone to manually run a script. The only real drawback I see to this approach is that you would have to create two triggers for every folder. This could end up being a lot of work, so if you go this route I suggest you keep the folder count to a minimum.
The Trigger
Run the following commands within MySQL
USE mythconverg;
DELIMITER $$
CREATE TRIGGER videometadata_insert BEFORE INSERT ON videometadata FOR EACH ROW BEGIN IF NEW.filename LIKE '%/<<FOLDER_NAME>>/%' THEN SET NEW.showlevel=4; END IF; END;
$$
DELIMITER $$
CREATE TRIGGER videometadata_update BEFORE UPDATE ON videometadata FOR EACH ROW BEGIN IF NEW.filename LIKE '%/<<FOLDER_NAME>>/%' THEN SET NEW.showlevel=4; END IF; END;
$$