Even though I have kids (12,12,13,15 yrs), I have given up on parental controls in MythTV.
It might work ok for videos, but I have not found a way to implement it for recordings that worked in a non-cumbersome way.
But a password for recording groups, which you already tried, is probably the best you can get. One thing I have on my LinHES box, to make selection of recording groups simpler, is to always show recording groups. This way you do not have to press "M" to change the recording group. You can change this behaviour in the MythTV settings.
We have recording groups for "Action films", "Kids series", "Kids Movies", "Drama", "Comedy" and a few more. It is simple to set a password for each group the kids are not supposed to watch and simple to switch between groups, but having to enter the password each time was too cumbersome.
So now our solution is to log the use of the box in a file and to tell the kids not to watch outside assigned hours. This works most of the time
jens