It depends on what you mean here. If you mean having chosen a rock name they've got an avatar they want to change, the good news is that everyone starts with that avatar no matter what rock name they chose. All they need to do is change the eyes, lips and hair to make it more to their taste.
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If you mean they've chosen a rock name and they want the chance to choose again, then their teacher can clear the chosen rock name so that the next time the child logs in they can choose again. (Teachers - go to Account > Classes tick the box next to the child's name and press the Reset Pupil Data Options button and choose rock name).