DVD parental controls

I have a zenith DVB412 dvd player. There is not enough information in the operating guide for setting up parental controls. There seems to be no consistancy. Some PG rated movies require a password and some R rated movies do not. As a parent how can I be sure that I am using parental controls correctly? It's worrisome when R rated movies seem so available to the kids but I have to put in a password so they can watch Two Brothers!