My neighbor went through this with her daughter in kindergarten. There was another girl in the class who talked about being fat (she was thin) because she heard her Mom talk about it constantly. The other girls in the class picked up the idea and ran with it. My friend brought it up to the teacher, and eventually the school counselor got involved. They had a "nutrition and exercise" talk at school with the entire class and invited parents as well. They also talked about body image with the girls. Even at that young age, they are aware of what we idealize, but not as aware of the drawbacks of being too thin. Maybe you could check with the parents of your daughter's friends and see if they are having the same issues. It is easier to work as a team than attack one at a time.