TypeError: __init__() missing 1 required positional argument: 'on_delete' happens when your foreign key field doesn't have on_delete argument.
As you know, from Django 2.0 on_delete is required.
Solve The Issue
To solve this issue, you need to add the on_delete parameter.
parent = models.ForeignKey(Parent, on_delete=models.CASCADE)
Check the document for more details.