The fact these innocent-looking children can make another person do anything with the power of their mind. Like by making someone still their hand into boiling water (Mara does this to her mother Barbara), or cut themselves open (the kids do this together to Dr. Susan Verner). The victim is aware of it the entire time.
After the entire village passes out, they awaken to discover the charred upper body of one man who had collapsed on top of a burning outdoor grill.
The 'remains' of the stillborn a.k.a. David's "twin". An alienesque bloated thing floating in a tube of liquid. Terrifying.
Dialogue states that this has happened before. It could always happen again.
And in at least two of the instances, there weren't just ten children; there were twenty or even thirty children born at once. In the latter instance, none of them survived very long after birth, but still.
When Mara overclocks her powers at the climax to punch through her father's mental blocks, it shows on her face, which goes from just eyes glowing to her full face glowing in a pattern that makes it look like there is something pretty damn inhuman just under the skin...
