The parental controls built into the Mac OS X Mail client can be easily bypassed by anyone who knows the email address of the child and his/her parent.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/93398/macosxparental-bypass.txt
Source: https://packetstormsecurity.com/files/93398/Mac-OS-X-Mail-Parental-Controls-Vulnerability.html