Add a new CNAME record on your local DNS server for your local Google domain(s) pointing to forcesafesearch.google.com.
Create a CNAME record for www.google.com that points to forcesafesearch.google.com.
NOTE: You may need to add a www.google.com zone and provide an A record override for www.google.com to 216.239.38.120 (forcesafesearch.google.com).
Clear your DNS cache of the saved www.google.com record.
Perform a search for something.
Confirm it is working by searching for the word boobs.
This should fail.
It should also show a message that Safe Search filtering is on.