How to restrict movies in my iTunes library from being watched by anyone?

I want to restrict the ability of my children to watch R-rated movies that are in my iTunes library. Ideally, I would like a password prompt to be displayed when I or my kids click on an R-rated movie. Is this possible? Is there any other way to do it? I understand that the iTunes store has parental control options. I use these options as needed. Right now, I'm concerned with parental controls for the iTunes library.