We created a WordPress (installed on our server) blog which requires logins we have to approve. We share this with family members, with an email notification to them when something new is posted. They can post comments to the site.
We really actually did this for ourselves, as a kind of family photo album/blog, and so would have it even if no one else was invited :-)
Have you tried cloning the app and making adjustments to it? https://appcloner.app/
How else will they continue to exchange illegal images with each other?