Sexual abuse may also be occurring in children with physical or emotional abuse. Tell them that you believe them. It is not their fault no matter what the circumstances. You know other children to whom this has happened, it is not just them. You will try to stop it happening.
Adults sometimes do wrong things.
They were right to tell. Do not make any promises that cannot be kept, particularly about keeping information secret. Try to remain calm. Listen to the child's story. Make the child comfortable while you organise things.