code: protected String getChildName(String child, String parent) throws IOException {
      if (child.startsWith(parent)) {
        return child.substring(parent.length());
      }
      throw new IOException(ExceptionMessage.INVALID_PREFIX.getMessage(parent, child));
    }
docs: Gets the child name based on the parent name.
  
  @param child the key of the child
  @param parent the key of the parent
  @return the child key with the parent prefix removed