I have two kids. I asked people to use signal to send and receive the photos. Asking people to follow your requirements only works for the direct immediate communication. The photos of my kids were sent by the recipients I sent them to (over signal) to other members of the family, over gmail (unencrypted), WhatsApp, Instagram, etc. I learned that years after.
This was in direct violation of my express requests. When I confronted them, they played dumb.
So, not to be a buzzkill here OP, but if you did this to get more people to use your messenger of choice, good job, it worked. If you did this so the pics of your kids stayed on safe apps, don't fool yourself. They didn't.