所以我想出来了：我在wp config中添加了默认主题：

// Setting default theme for newly created sites 
define( \'WP_DEFAULT_THEME\', \'theme-child\' );

# ON SIGNUP OF NEW SITES A NEW PAGE IS CREATED CALLED "Homepage" AND SET AS 
THE STATIC FRONT PAGE

add_action( \'wpmu_new_blog\', \'process_extra_field_on_blog_signup\', 10, 6 );

function process_extra_field_on_blog_signup( $blog_id, $user_id, $domain, 
$path, $site_id, $meta ) {
switch_to_blog($blog_id);


$new_page_title = \'Homepage\'; //page title
$new_page_content = \'TEST\'; //content of page goes here
$new_page_template = \'\'; //page template.

$page_check = get_page_by_title($new_page_title);
$new_page = array(
        \'post_type\' => \'page\', 
        \'post_title\' => $new_page_title,
        \'post_content\' => $new_page_content,
        \'post_status\' => \'publish\',
        \'post_author\' => 1,
);
if(!isset($page_check->ID)){
        $new_page_id = wp_insert_post($new_page);
        if(!empty($new_page_template)){
                update_post_meta($new_page_id, \'_wp_page_template\', $new_page_template);
        }
}

$homepage = get_page_by_title( \'Homepage\' );
if ( $homepage )
{
    update_blog_option( $blog_id, \'page_on_front\', $homepage->ID );
    update_blog_option( $blog_id, \'show_on_front\', \'page\' );
}
restore_current_blog();
}