Groups WP taxonomies by parent

group-product-categories.php

foreach( get_terms( 'product-categories', array( 'hide_empty' => false, 'parent' => 0 ) ) as $parent_term ) {
    // display top level term name
    echo $parent_term->name;

    foreach( get_terms( 'product-categories', array( 'hide_empty' => false, 'parent' => $parent_term->term_id ) ) as $child_term ) {
        // display name of all childs of the parent term
        echo $child_term->name;
    }
}