Multiple options exist to enable and enforce safe searching at both application and account levels. If you want to enforce it over an entire network you will need to look at DNS records.
Each of these are interesting as you are overriding actual Internet DNS records. As such, none of these work if the parent domain is utilizing DNSSEC signed zones.
Enforce Google safe search by defining the CNAME below on your local network.
www.google.com CNAME forcesafesearch.google.com
Bing
Enforce Bing safe search by defining the CNAME below on your local network.
www.bing.com CNAME strict.bing.com
Youtube
Youtube uses a number of different domains but you would create the same CNAME over them all. Two modes are possible. “Strict” is the most restrictive while “Moderate” allows a larger number of content.
For Moderate Restricted YouTube access, CNAME each domain to restrictmoderate.youtube.com.
www.youtube.com CNAME restrictmoderate.youtube.com
m.youtube.com CNAME restrictmoderate.youtube.com
youtubei.googleapis.com CNAME restrictmoderate.youtube.com
youtube.googleapis.com CNAME restrictmoderate.youtube.com
www.youtube-nocookie.com CNAME restrictmoderate.youtube.com
For Strict Restricted YouTube, use restrict.youtube.com in the CNAME instead.
A test page can provide validation: Check Content Restrictions
DuckDuckGo
DuckDuckGo also needs a CNAME to enforce safe searching.
duckduckgo.com CNAME safe.duckduckgo.com