Context · Safety
Every app that promises messages disappear is describing its own design choice, not a law of physics, because the person on the other end can always photograph their own screen.
Disappearing is a feature, not a guarantee
Snapchat, Instagram vanish mode and WhatsApp view once all remove the message from the app. None of them can stop somebody screenshotting it, filming the screen with another phone, or simply showing it to the person sitting next to them, and kids treat all three as impossible because the app implies they are.
Deleting works on your copy only
When your kid deletes a photo they are deleting their copy. Anything already sent lives on somebody else's device and on the app's servers, so the honest sentence to a kid is that deleting helps and does not undo, and saying otherwise is the thing that makes them stop believing you.
The rule that actually holds
Every filter and every setting fails eventually, so the only durable protection is a habit: send it as if the whole grade might see it. Kids find that easier to hold than a list of app rules, because it works the same way on whatever app arrives next year.