I don't really see it in that way? As I got it, the child never made their age clear from the begining, which they should have done (I'm not blaming the child though) It's totally impossible to know if someone is an adult or child. And since the child was sending stuff that would be expected to be from an adult, it's natrual for Moss to have assumed that the person was an adult.
I really don't mean to argue, I just want to say my opinion.