How can I force Google SafeSearch on my kids' Mac?
I did it with Squid and Squidguard.  Add the safe parameter to the URI.  All computers in the house got their DNS and web proxy from that server.

Even this can be defeated by using https to go to a proxy that serves the pages for you.  Then "google" is only in the encrypted packets, not in the URI that your local system sees.

You can block known proxies, but new ones are frequently created.  And Tor Browser can run from a USB drive.

How sophisticated are the kid's tech skills?  Are they actually trying to bypass filters?  If the answers are "very" and "yes" then you can disconnect internet completely and they'll still do it by going to a friend's house.