What is the worst thing a parent can do to their children

Create in them a sense of worthlessness. To make a child believe that they don't matter and are inconsequential. To discover that their love is not reciprocated. This will have long lasting effects, well into adulthood, manifesting as chronic despair and diminished self worth. It teaches the weeping willow how to cry and the clouds how to cover up a clear blue sky.

Mother is the word for god on the lips of children.