hi,
i used to do this when i was anorexic...it often happened when i would start eating food, then freak out and decide i couldn't eat it so i would spit it out. it is possible for issues with food to start at such a young age - in fact, my own food issues started at eight - by that age i already had distinct foods that were off limits, and could tell you the calorie, fat and sugar count of the majority of food i ate. i do not think it had anything at all to do with body image or weight either - my physical appearance is in fact something that i have always been completely satisfied with and an aspect of myself that i am extremely confident about. i used food as a way of controlling my parents (amongst other things), as me not eating was one of the few things that would get their attention. so maybe have a think about how things are going for her and home, school, or any other area of significance in her life. a chat with a doctor or child psychologist might be an idea - it could just be bad behaviour that needs disciplining, but then again it could be a sign of more serious things, so it pays to be on the safe side!