Overview
Never Trust Children is an unmarked secondary quest in The Witcher 3: Wild Hunt, with one part in Novigrad and one in the Skellige Isles. Both are proximity based and vanish if you leave, so do not go looking for them at a low level. The Novigrad part must be over before Hidden Messages of the Nilfgaardian Kind becomes available.
Novigrad
Beside the Crooked House in The Bits, a child is crying. When Geralt approaches, the child runs off a short way, right to where bandits are lying in wait. Once they are dead, the child is gone.
Skellige
On the road between Arinbjorn and Sund, a boy calls for help and says a bear attacked his father while he was cutting wood. He steps off the road and bolts as a gang of thugs springs the trap. Again, the child is nowhere to be found afterward. Here you can also find the thugs first and skip talking to the boy.