How to add new sidebar widget area to child theme?

Well, this is what worked for me. In my functions.php file I put the following code:

function header_widgets_init() {

register_sidebar( array(

    'name' => 'Header Sidebar',

    'id' => 'header_sidebar',

    'before_widget' => '<aside class="widget %2$s">',

    'after_widget' => '</aside>',

    'before_title' => '<h2 class="widget-title">',

    'after_title' => '</h2>',

) );

}

add_action( 'widgets_init', 'header_widgets_init' );

…and in my header.php file I used:

<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('header_sidebar') ) : endif; ?>