For me, there's two things - the unknown and the innocence, sprinkled with 'rumors'
Just imagine, you're in an alley at night, it's raining slightly, and you hear a voice behind you. You turn around, and it's a kid - six years old at most.
"Have you heard of the Wilted?"
(roll for history, make the DC impossibly high). You don't know about the Wilted.
"No? What is that?"
"Mommy says they come out at night, the moment every flower has closed back up, and roam the streets until the sun makes them bloom again. If they find you, they make you wither away, like an old blossom with not enough water." (Got the idea from the Wither and Bloom spell, in this case)
"What do they look like?"
"I don't know. Nobody does. All we know is every morning, in the town square, there is a new body, all dried up, on display and everything."
"Then why are you out alone at night?"
"Because I can't find Mommy."
Something like that is kind of what I like to reach for with anything meant to be unnerving. Maybe the child itself is the monster, but don't make it obvious.
