How to display child categories of all parent category within a singe loop?

Here you go:

$args = array(
        'post_type' => 'listing',
        'order' => 'ASC',
        'hide_empty' => false,
        'parent' => 0,
    );

    $listCatTerms = get_terms( 'listing-category',$args);
    if ( ! empty( $listCatTerms ) && ! is_wp_error( $listCatTerms ) ){ 
        echo "<ul>";   
        foreach ( $listCatTerms as $term ) {
            echo '<li>'.$term->name.'</li>';
            $child_terms = get_categories( array(
                'parent' => $term->term_id,
                'hide_empty' => 0 ));
            //var_dump($child_terms);
            echo "<ul>";
            foreach($child_terms as $child_term){
                echo '<li> -- '.$child_term->name.'</li>';
            }
            echo "</ul><br>";
        }
        echo "</ul>";   
    } 

This should display the name of parent category and all its child category after.