filtering on From with postfix / postgrey

[Philip <subs%christiantena.net@localhost>]

I have three small children who would like an email account to email
their friends.
I have my own domain which points to a netbsd 3.1 mail server running
postfix, postgrey and dovecot.
What I would like to do is set up email accounts on the server for the
children which will only receive email from a white list of From:
addresses, but which will not affect other users normal accounts.
I guess I don't need a whitelist per child if that makes any difference.
I can't figure out how to do this, does anyone here know how to?

thanks, Philip