Context · Safety
AirDrop and its equivalents let any nearby phone send a picture to your kid's phone without knowing their number, which is mostly harmless and occasionally the way an unpleasant image arrives.
How it actually works
It sends over a short range direct connection rather than the internet, so the sender only has to be within a room or a bus of your kid. No contact details are needed, which is exactly what makes it convenient between friends and what makes the other use possible.
The setting that decides everything
If receiving is set to everyone, any nearby phone can send a preview that appears on the screen whether your kid wants it or not. Set to contacts only, it goes back to being a useful tool between friends, and this is a thirty second change with a large effect.
Why kids do not mention it
A picture that arrives unrequested feels to a child like something that happened to them, and the first instinct is usually to assume they will get blamed for having it. Saying in advance that an unwanted picture is never their fault is what makes them mention the next one.