code: def get_child_value(parent, name, allow_missing=0):
          
          if not parent.hasElement(name):
              if allow_missing:
                  return np.nan
              else:
                  raise Exception('failed to find child element %s in parent' % name)
          else:
              return XmlHelper.as_value(parent.getElement(name))
docs: return the value of the child element with name in the parent Element