docs: 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.")