I know it is a disturbing idea for a fic but...
What if the reader gets reincarnated as parent of a serial killer in a horror game-novel? Sure they'll manage to end up changing the course of lives of the many victims of the child but...the soul if the child is doomed from birth, and in the process of aviating from the fate of a killer, it latches onto the life of the parent?
Like the idea of a yandere child is interesting to me, a child that loves their parent to the point of obsession (nothing sexual, completely platonic), and the parent unknowingly becomes the center of a dark world.
You looked at the bundle in your arms, staring into the eyes of the babe staring back at you. An infant wouldn't have the ability to even have vision at that age but...it seemed the babe was staring into your soul, so this was going to be the one that becomes a cult leader that destroys many lives huh? And to think your life was stuck with the babe...in the original time line you'd die by the hands of this child, six years from this moment onwards so...why...why the universe had to play with you like this?! You had frowned, keeping the bundle close to your chest while deep in thought, not noticing the gummy smile of the infant, you didn't notice, your willingness to hold the bundle close, had already changed the course of the story, because it was said the child was "unable" to smile.
"Mommy/papa, have I told you I love you so much?" *the three year old said in perfect speech while you tucked them in, looking up at you with a soft smile, those round eyes, staring into your soul* "Never leave me, promise? Promise?" *the child paused before muttering* "Because if you do I'll follow you around"