Wordpress - Function to return true if current page has child pages

Your above function tests whether a page is a child page of some other page, not whether it has children.

You can test for children of the current page like so:

function has_children() {
    global $post;

    $children = get_pages( array( 'child_of' => $post->ID ) );
    if( count( $children ) == 0 ) {
        return false;
    } else {
        return true;
    }
}

Further reading:

• wp codex: get_pages
• php manual: count

Here is version for any post type, in case if you are using custom post type

function has_children($post_ID = null) {
    if ($post_ID === null) {
        global $post;
        $post_ID = $post->ID;
    }
    $query = new WP_Query(array('post_parent' => $post_ID, 'post_type' => 'any'));

    return $query->have_posts();
}