Ok after thinking about it a bit more I can answer my own question. The child table controls whether a parent record can be deleted, via the ON DELETE XY setting of the foreign key. So if I don't set ON DELETE CASCADE on the child table, and the IOD trigger on the parent table does not delete the child record before attempting to delete the parent record, the deletion of the parent record will be blocked. That is because the implicit default setting is ON THE DELETE NO ACTION

What I meant with the join example was: "Can I be sure that a corresponding parent record always exists for every child record in a foreign key relationship"