(I am going to assume that as you are reading this, you already know the symptoms of narcissism, they have no empathy or feelings for others, are incredibly self centered, treat other people as objects and are happy to destroy the lives of those around them in order to take what they want for themselves.)
Sometimes the child is kept naive and vulnerable so that they believe they will be unable to survive without the narcissistic parent. This is called infantilization and the child relies on the support of the parent, no matter how awful their life may be with them. The parent, of course, loves all the attention the child gives them.
The opposite is parentification, where the parent leads the child to believe that they have to take care of their parents at all costs, be it financial, physical or emotional care. The child may have to be the parents therapist, or take one parents side against the other, lots of housework, paying the bills and so on.