Invisible to Adults is the premise of many stories aimed at children; because of their supposed "innocence," children are able to see things that adults are unable (or perhaps unwilling) to see. However, Fridge Logic sets in when you remember that the ones making these media are adults — adults who tend to have the kind of imagination necessary to come up with this kind of thing.
The basic idea behind the trope is that people become more closed-minded as they grow older, and are less likely to believe in the fantastical. In reality, most adults have much bigger imaginations than children; they just know how to separate fantasy from reality. In fact, adults are far more likely to understand religious and fantastical concepts than children because of their experience and knowledge.
There is some logic behind the trope, of course. People have trouble learning new concepts as they age, due to the way their brains work. However, the adage that "You can't teach an old dog new tricks" is simply not true — you can; it's just harder. And despite what the media (and some old geezers) will tell you, the urge to partake in the fantastical never quite leaves most people; this is evident by the fact that we still enjoy listening to stories, even though we know they're fake. Never mind the fact that, as we grow older, we enjoy types of stories that we never would have enjoyed, much less understood, as children.