Get the first children URN for a given resource

        :param reference: CtsReference from which to find child (If None, find first reference)
        :type reference: CtsReference, str
        :return: Children URN
        :rtype: URN

def getFirstUrn(self, reference=None):
        """ Get the first children URN for a given resource

        :param reference: CtsReference from which to find child (If None, find first reference)
        :type reference: CtsReference, str
        :return: Children URN
        :rtype: URN
        """
        if reference is not None:
            if ":" in reference:
                urn = reference
            else:
                urn = "{}:{}".format(
                    str(URN(str(self.urn)).upTo(URN.NO_PASSAGE)),
                    str(reference)
                )
        else:
            urn = str(self.urn)
        _first = _SharedMethod.firstUrn(
            self.retriever.getFirstUrn(
                urn
            )
        )
        return _first