Maybe two possibilities will offer stimulus for you to invent more ideas of your own. Character watches her child with concern and notices numerous lapses in memory and behavior, being a responsible parent she has her child examined and tested only to find that she has a brain tumor. These kinds of issues add dramatic effect to a story line. You can then decide how to resolve the crisis, tragedy or happy ending, are two traditional resolutions to this type of story. Or the daughter, an only child, has created an imaginary friend who she is so invested in socially and emotionally that nothing else really holds her interest. Depending on the kind of story you want to tell, you could create ghost for a Gothic approach or make it about everyday reality and have the mother find a way to wean he from her fantasy and introduce the child successfully into the world of her schoolmates.