This small tool implements a whitelist on a Postfix mail-server. It prevents certain recipient addresses (your kids ones) from receiving mail from any not whitelisted address. Any mail from not whitelisted senders is redirected to a delegate (a parent). Code is here http://files.a385e-5.de/files/ChildProt-0.9.tar.gz Configure it by adding this line into the master.cf of the Postfix installation: 
childprot unix  -       n       n       -       25      spawn user=mail argv=/opt/sbin/ChildProt
and this to the main.cf: 
smtpd_recipient_restrictions =
  [...]
  check_policy_service unix:private/childprot
  [...]
The restricted recipients and the whitelists are stored in an SQLite3 database: 
CREATE TABLE child_address_t (
  child INTEGER REFERENCES child_t(id),
  address TEXT
);

CREATE TABLE child_t (
  id INTEGER PRIMARY KEY,
  name TEXT,
  delegate TEXT
);

CREATE TABLE whitelist_t (
  child INTEGER REFERENCES child_t(id),
  address TEXT
);

CREATE VIEW child_v AS
  SELECT c.id as id,
         c.delegate as delegate,
         ca.address as address
  FROM child_t c,
       child_address_t ca
  WHERE c.id = ca.child;
Restricted persons together with their delegates are added to the table child_t, multiple addresses can be assigned to those persons in child_address_t. Whitelists per person are maintained in whitelist_t. The tool is querying the view child_v.