Serialize the dictionary and append all serialized children to the element.

def _serialize(
            self,
            element,  # type: ET.Element
            value,  # type: Dict
            state  # type: _ProcessorState
    ):
        # type: (...) -> None
        """Serialize the dictionary and append all serialized children to the element."""
        for child in self._child_processors:
            state.push_location(child.element_path)
            child_value = value.get(child.alias)
            child.serialize_on_parent(element, child_value, state)
            state.pop_location()