The First Amendment is an important protection.  However, exceptions have been made to protect children.  
The California law did not ban sales of video games to children, it imposed a fine on sellers who sold to people who were younger than the age rating of a game.  The reason for the law was that a great deal of research indicates that playing violent games does increase aggression for some people, and also may desensitize a person to the consequences of violence.  As a society, we limit children from exposure to harmful things like pornography.  In my opinion it was reasonable to try to put teeth into the game ratings system.

Jeanne Brockmyer, Ph. D. education.com expert clinical child psychologist