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.