There are no such classes in J2SE.  If such exist in
you IDE, try either..
A) F1, or ()perhaps more productively, ..
B) <http://java.sun.com/docs/books/tutorial/>

Try RTFM first.

Just have the parent form keep a reference to the child form in a member
variable. Then the [J]MenuItems can get this reference to the child form
and do as they please.

public class ParentForm extends JFrame {

    private JFrame childForm;

    public void showChildForm() {
        childForm = new JFrame();
    }

}

You should not use a parent-child relationship, in my opinion.

Make the windows separate objects.

Then, in your "parent" window, leave actionListener methods exposed so
you can install methods at runtime.

public MyMainApplicationWindow extends JFrame {
  private JMenuBar menuBar;
  private JMenu helpMenu;
  private JMenuItem aboutMenuItem;

  public MyMainApplicationWindow() {
    super( "My App Name");

    menuBar = new JMenuBar();
    helpMenu = new Jmenu( "Help" );
    aboutMenuItem = new JMenuItem( "About My App..." );
    helpMenu.add( aboutMenuItem );
    menuBar.add( helpMenu );
    this.setMenuBar( menuBar );

    this.pack();
    this.setSize( 200, 200 );
    this.setLocationRelativeTo( null );
  }

  public void setAboutAction( ActionListener a ) {
    aboutMenuItem.addActionListener( a );
  }
}

Now you can choose at run time what "About My App" does.  This is
important for testing.  In a small app (homework) it won't matter, but
anything larger you'll want to test these two windows separately.
Binding them at runtime is one way to ease your testing.

Note: code not compiled.