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.
No one cares. Nobody around you understands the security, the need for it, and the requirements. They will pretend, to see your kid. And then immediately and completely stop caring. It works for making people adopt your favourite messenger, yes. But nothing else.