Get the next child PID in the PID relation.

def next_child(self, child_pid):
        """Get the next child PID in the PID relation."""
        relation = self._get_child_relation(child_pid)
        if relation.index is not None:
            return self.children.filter(
                PIDRelation.index > relation.index
            ).ordered(ord='asc').first()
        else:
            return None