Well, your elementary students will eventually get EMail, correct? Why not add them to LDAP using email@example.com. Your Moodle server won't be setup with email accounts for students and if emailing is attempted it will fail/be rejected. All Moodle checks is the format of that field ... username@someaddy it does not actually check the validity of the user/addy.
If you do it this way, elementary students should not be subscribed to any forum. Even if they are, errant mail will go to the moodle server itself and be rejected. Stops errant EMail from flooding your true Mail server with junk and keeps that errant traffic to the Moodle host alone.
Later, as they grow to an age where EMail is allowed, one just needs to update all the LDAP objects.
'spirit of sharing', Ken