My colleagues at Space-O Technologies, who are mobile app developers, recently developed Gallery Guardian, a parent guardian app. In fact, this app has been featured on BBC, The Telegraph, and Daily Mail.
I tried to share the links, but as of a new user here, I'm unable to share the links.
Anyway, Google "Gallery Guardian"
You will find it out on the top of the result.
Lemme share you the screenshot of the app so you can recognize it.

And this is how it works:
- Install it on both on a parent and child's device
- Pair the two together
- After an initial sync and scan procedure, if the app detects a suspicious image it alerts the parent.
- No images are shown or sent to the parent.
- The parent then takes the appropriate action.