What point is Shelly trying to make about parents and children in the book? My teacher keeps talking about it, and I am having trouble.

There is a very good reason your teacher keeps talking about this.  Let me outline some things for you:

1. Mary Shelley's own mother died from complications of giving birth to her.  2.  There are lots of parent-child relationships in the book.  To name a few:  Caroline Beaufort (Frankenstein) and her parents, Victor and his parents, Elizabeth and her childhood, Justine Moritz and her childhood, Safie and her father, Felix and Agatha and their father, Adam and God, and of course, the Creature and his "father".  3.  The underlying message that Shelley is trying to get across is this:

If you are going to bring a child into the world, you have to take responsibility for that child.  Clothing, food, shelter, education, and love are among the necessities.

The Creature is denyed all of this, and as a result, he feels angry and abandoned.  (Not unlike the feelings Mary Shelley had growing up without her mother.)

The parent-child relationship is a very important theme in the book, and understanding the message Shelley is sending will shed greater light on her message of unconditional love.  Had the creature been given this basic element, things would have turned out very differently for everyone.