The [uncanny valley](https://en.wikipedia.org/wiki/Uncanny_valley) and unexpected behavior. You see something that looks and acts *almost* like a normal child but you clearly see that something is very, very wrong. That contrast between “children should be cute and harmless” that your brain expects and “it’s not a normal child and it’s not harmless” that you see creates creepiness. When you see a monster, you at least know what to expect.
There’s a joke: “In a bad horror, the screen is filled with monsters and blood but you’re not afraid. In a good horror, nothing is happening, then a little hedgehog slowly enters the scene and you run to change your pants.”
It seems like you might not have had a kid tell you about their invisible friend and they are dead serious about it. Or had them tell you any of their random nonsense. ex. [https://twitter.com/advancebase/status/1194437674901393408](https://twitter.com/advancebase/status/1194437674901393408?lang=en)
But there’s also probably a bit of cuckoo effect thrown in. You’re trying to protect your/a small child but “ah, it’s actually a ghost!” Similarly it gives the protagonists a reason to let a potential threat into their social circle/space.
Additionally, the audience “least expects” a child to be the largest potential threat(, and it would be surprising if it weren’t a trope). It subverts expectations. It just doesn’t work the same way if it’s cliche. It was probably super great the first dozen times.
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