Morgana, it is difficult to answer your grandchild's question. I just wonder if somebody maybe at a toddlers group may have said something to make her aware? I remember reading in a newspaper about a little brown girl who put bleach on her skin. I suggest that your daughter should cultivate friends who are a similar colour to her daughter so that she can see that being brown is a natural thing and perfectly OK in this country, at least to most (not sadly in America.)
My husband was of Indian origin and he was very good looking, as are both my children, married to white English people, and all my grandchildren . One of my grandsons takes after me, fair skinned and reddish hair, another grandson is fair and was white-blond at first but now had brown hair, one granddaughter has brown hair and grey eyes, and the other has black hair and creamy complexion.
My daughter was teased at school, called names, and she asked me why I had married my husband instead of a man with a pink face. I think she is still embarrassed about being half Indian. But her husband told her that Indian girls are the most beautiful in the world.
Its all genetic and one day everyone will all look the same.