I doubt he's a child maybe an adult stuck in a child's body or he just stopped growing but until we see and background story or a flash back we can't know for certain, but he was crazy before they entered that world, and showed signs he was controlling the dude even from the game screen so I can see why most people don't start automatically thinking of giving the grand duke therapy or a loving figure.
My own guess he's so far gone in the head that he'll never be able understand such things no matter how much you try to explain it, like the inquisitor guy from sss class suicide hunter.
Same small size dude with strong abilities but the inability to comprehend normal human thoughts and emotions with his first thoughts to actually kill people most of the time when he's giving a task ╮( ̄▽ ̄)╭
The fact that y'all INSTANTLY go to violence against a CHILD, instead of thinking about therapy, mental ward, a stable loving parent figure, etc.
Like, eat grass.
As in smoke some weed and CHILL