You asked the question, "why do little girls do this?" -- Because the big girls they live with allow them to! I have seen so many moms pretend they don't notice when their child is being nasty to other children. Why was the grandmother ASKING the little girl to be nice to your daughter? Why wasn't she using this moment to explain how to treat other people, and how she would like to be treated? And possibly TELLING her how to treat other children. Yes, many adults forget that they actually are supposed to tell children when they are behaving in an inappropriate (or mean) manner. When adults are passive to these behaviors they are giving tacit consent. I am so sorry that your daughter had to experience this at the age of three. Unfortunately, those 5 year olds are the 12 year olds of the future. I, personally, don't think it would be such a terrible thing for you to say, if the situation arises again, something to the effect of, "I know that hurts your feelings - that's why Mommy and Daddy always remind you to be nice to others - because I know a nice girl like you would NEVER want to hurt anyone's feelings." If you're feeling particularly Momma Bear - don't be afraid to add, "Unfortunately, not all mommies do that."