Use the following PHP code in archive.php
<?php
$term = get_queried_object();
$term_id = $term->term_id;
$taxonomy_name = $term->taxonomy;
$termchildren = get_term_children( $term_id, $taxonomy_name );
echo ‘<ul class=”sub-category-list”>’;
foreach ( $termchildren as $child ) {
$term = get_term_by( ‘id’, $child, $taxonomy_name );
echo ‘<li><a href=”‘ . get_term_link( $term, $taxonomy_name ) . ‘”>’ . $term->name . ‘</a></li>’;
}
echo ‘</ul>’;
?>