Two workarounds, depending on how many parents you have and in how many courses. (Whatever you do I wouldn't enrol them into student courses)
- make a course just for parents. Add them with the regular student role. Then message them from there. If you really need to filter the parents from different courses, make groups with the course names so then you can filter the parents by group before messaging.
- Edit either their first or surname (or even username) to include the word Parent - and then you can use Bulk user actions to filter from therE.
(And by the way - we are not yet on Moodle 3.5.2)