How to Unregister Footer Widget from Parent Theme

I have two footer widgets in my parent theme that I need to unregister, so that I can register 3 new footer widgets in my child theme. (The child theme footer needs to be 3 columns.) This is what is in the parent theme:

// Create Footer Left widget area.
	$sidebars = array('footer-left');
	foreach ($sidebars as $sidebar) {
    register_sidebar(array('name'=> $sidebar,
    	'id' => 'footer-left',
    	'name' => __('Footer Left', 'slamtheme'),
		'description' => 'This widget controls the left footer column.',
        'before_widget' => '<div class="large-6 large-uncentered medium-6 medium-uncentered small-12 small-centered columns footer-left"><article id="%1$s" class="widget %2$s">',
        'after_widget' => '</article></div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>'
    ));
	}

	// Create Footer Right widget area.
	$sidebars = array('footer-right');
	foreach ($sidebars as $sidebar) {
    register_sidebar(array('name'=> $sidebar,
    	'id' => 'footer-right',
    	'name' => __('Footer Right', 'slamtheme'),
		'description' => 'This widget controls the right footer column.',
        'before_widget' => '<div class="large-6 large-uncentered medium-6 medium-uncentered small-12 small-centered columns footer-right"><article id="%1$s" class="widget %2$s">',
        'after_widget' => '</article></div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>'
    ));
	}

I put this in my child theme…

//Unregister Footer Widget
add_action( 'widgets_init', 'unregister_widgets', 11 );

function unregister_widgets() {

    // remove (some) WordPress default Widgets
    unregister_widget( 'footer-left' );
    unregister_widget( 'footer-right' );

}

…but it did nothing. What am I doing wrong? How can I unregister these footer widgets?