A simple script, intended to be run in the context of a WordPress install -- `wp-cli`'s `eval-file` is the easiest way -- to ping the cron of all sites registered in the network.

execute-cron-on-all-sites.php

<?php

$start_date = date_i18n('Y/m/d H:i:s');
$start_time = microtime(true);

echo "Start site-wide wp-cron at $start_date\n";

$sites = wp_get_sites();
foreach ($sites as $site) {
  $url = "http://" . $site['domain'] . $site['path'];
  echo "Ping wp-cron.php for `$url` ... ";
  file_get_contents($url . 'wp-cron.php');
  echo "OK.\n";
}

$end_time = microtime(true);
$end_date = date_i18n('Y/m/d H:i:s');

$time = ($end_time - $start_time);

echo "Finished at $end_date ($time s)\n\n";