Apple - How do I restart the cron service on OSX?

The cron daemon is managed with launchctl.

If you insist on restarting it, you have to unload and load the respective launch daemon:

sudo launchctl unload /System/Library/LaunchDaemons/com.vix.cron.plist
sudo launchctl load /System/Library/LaunchDaemons/com.vix.cron.plist

But: adding or removing arbitrary crontabs shouldn't require a reload of the daemon to enable or disable them.

The above launch daemon plist contains a Watch Directory directive which detects (crontab) file creation and deletion or modified dates and "instructs" cron respectively.

From the cron man page:

The cron utility is launched by launchd(8) when it sees the existence of /etc/crontab or files in /usr/lib/cron/tabs. There should be no need to start it manually. See /System/Library/LaunchDaemons/com.vix.cron.plist for details.

So, interstingly enough, cron is launched by launchd. However, according to man crontab

(Darwin note: Although cron(8) and crontab(5) are officially supported under Darwin, their functionality has been absorbed into launchd(8), which provides a more flexible way of automatically executing commands. See launchctl(1) for more information.)

So, cron which is launched by launchd, has been deprecated for launchd. Using cron is like wrapping bacon in bacon because you want bacon flavored bacon.

As far as restarting cron, there is no need. Once it sees a crontab file, it will automatically launch the job.

However, it's highly advisable that you start scheduling jobs using launchd.

sudo pkill -f cron

The LaunchDaemon has a keepalive section in it that will immediately restart it.