I may have found an easier way...
Is SpamAssassin implemented at the domain or at the POP box level? If the latter, I'm in business by creating a manual whitelist and a manual blacklist for each of my kids, assuming that SpamAssassin manual whitelist can override a manual blacklist.
whitelist_from you@ yourdomain . com
whitelist_from me@ mydomain . com
blacklist_from *
This would limit my kid's email to receiving from you and me and EVERYONE else would be flagged as spam and deleted.
If a manual whitelist can override a manual blacklist at the POP level, I'm in great shape with the tools TCH already has. Any SpamAssassin configuration gurus here?