// Filter to exclude a page and it\'s child pages from search query
function myfilter_exclude_pages_from_search( $query ) {

    // Save the post id of the page to be excluded in $parent_post_id_to_exclude
    $parent_post_id_to_exclude = 111; // Replace 111 with actual page id
    // Convert the post id to an array
    $parent_post_id_arr = array( $parent_post_id_to_exclude ); 

    // Apply filter only if this is a search query and not on admin panel
    if ( $query->is_search && !is_admin() ) {

        // Search only for pages
        $query->set( \'post_type\', \'page\');

        // Exclude the page id
        $query->set( \'post__not_in\', $parent_post_id_arr );

        // Exclude all child pages of selected page id
        $query->set( \'post_parent__not_in\', $parent_post_id_arr );
    }
    return $query;
}
add_filter( \'pre_get_posts\', \'myfilter_exclude_pages_from_search\' );

WP\\U查询参数参考：https://developer.wordpress.org/reference/classes/wp_query/