echo substr(wp_list_pages('sort_column=menu_order&child_of=' . $post->ID .'&echo=0<strong>&depth=1</strong>&title_li=<h3 class="page-links-title">' . get_the_title($post->ID) . '</h3>'), 20, -5);
}
i solved the problem by adding &depth=1 after &echo=0