Wordpress - List child pages for sub-nav

// ECHO SUB-NAV

function echo_sub_nav($postID,$forceCurrent=false){
    if($forceCurrent){
        echo '<style type="text/css">#sub_nav ul li.page-item-'.$postID.' a{color: #b30f21;}</style>';
    }
    if(is_subpage($postID)){
        $tempPost = get_post($postID);
        $subPages = wp_list_pages('title_li=&echo=0&child_of='.$tempPost->post_parent);
    }else{
        $subPages = wp_list_pages('title_li=&echo=0&child_of='.$postID);
    }
    if(!empty($subPages)){ ?>
        <nav id="sub_nav">
            <h2><?php if(is_subpage($postID)){
                echo get_the_title($tempPost->post_parent);
            }else{
                echo get_the_title($postID);
            }?></h2>
            <ul>
                <?php echo $subPages; ?>
            </ul>
        </nav>
    <?php }
}

// DEFINE SUB-PAGES

function is_subpage( $postID = null, $page = null ){
    $post = get_post($postID);
    // does it have a parent?
    if ( ! isset( $post->post_parent ) || $post->post_parent <= 0 )
        return false;
    // is there something to check against?
    if ( ! isset( $page ) ) {
        // yup this is a sub-page
        return true;
    } else {
        // if $page is an integer then its a simple check
        if ( is_int( $page ) ) {
            // check
            if ( $post->post_parent == $page )
                return true;
        } else if ( is_string( $page ) ) {
            // get ancestors
            $parent = get_ancestors( $post->ID, 'page' );
            // does it have ancestors?
            if ( empty( $parent ) )
                return false;
            // get the first ancestor
            $parent = get_post( $parent[0] );
            // compare the post_name
            if ( $parent->post_name == $page )
                return true;
        }
        return false;
    }
}