Overall, a very good extension. Could you add https://www.onesearch.com/ to the list?
edit: I think I found the answer to your question. In the url of the search result, there is a vm parameter that can have 3 values. I found out that vm=r is strict, vm=i is moderate and vm=p is off.
I hope this helps you.
edit2: Added one star after onesearch was added. Thank you again.

Thank you so much for this !!!

Thank you so much.....

Awesome! I love that I can still watch YouTube, but why do I have to choose "under 18" to allow restricted content? (moderate and strict block restricted videos)

It will be better if we can lock safesearch in android too

Great, especially when combined with another Add-on called "Public Fox", which allows adding password protection on the Add-ons menu to prevent disabling the Add-on.

Thank you so much for making this add-on. It is too easy to turn off safe search, and making it just a little harder is a great idea.

I really like this extension. It work very well. It enforce the strict mode on many search engines. An the user cannot change the strict mode. However, the extension should be password protected to block it's deactivation on a child account.