That's not necessarily true. In all likelihood we will see maybe a montage or something of the preparations that Ron and Hermione went through to ensure they were not going to leave their families in danger. We do hear a fair chunk about Hermione's parents at the start of the book as she explains what she has done, and that is probably what is being filmed.

Whether its a montage at the beginning, or a flashback, it sounds like a nice idea to have her mum and dad in there, we always see Ron's parents and never Hermione's. If they are mentioned in the book, put 'em on the screen!