Fem!Child!Reader x Nightmare Sans

There's a tale many people talk about from a particular tree on a hill. It was told to young Y/N who is only three years old and wants to know much more about this story about two skeleton brothers who guard their own side of their tree, one named Dream and the other Nightmare. Y/N seemed more focused on Nightmare though, they think this skeleton just needs help and is lonely. One day Y/N is left with a careless babysitter and Y/N goes off by themselves