Add the given PID to the list of children PIDs.

def insert_child(self, child_pid):
        """Add the given PID to the list of children PIDs."""
        self._check_child_limits(child_pid)
        try:
            # TODO: Here add the check for the max parents and the max children
            with db.session.begin_nested():
                if not isinstance(child_pid, PersistentIdentifier):
                    child_pid = resolve_pid(child_pid)
                return PIDRelation.create(
                    self._resolved_pid, child_pid, self.relation_type.id, None
                )
        except IntegrityError:
            raise PIDRelationConsistencyError("PID Relation already exists.")