Additionally, though I haven't tested it, this same method can be used to force Google Safe Search. Though it obviously doesn't help with any other search engines.
- Add a new CNAME record on your local DNS server for your local Google domain(s) pointing to forcesafesearch.google.com. For this example, we will be using www.google.com. Create a CNAME record for www.google.com that points to forcesafesearch.google.com.