Unfortunately, your child and I are not making as much progress as I hoped. As such, I no longer find it constructive to continue the tutoring sessions. If you like, I can continue through [explicit date]. Or stop sooner if you prefer. If you would like to find another tutor, consider [resource for finding tutors].
This avoids putting the blame on the child explicitly. So if the parent chooses to believe the problem is you while you believe the problem is with the child or parent, your conflicting beliefs never clash. For the same reason, this avoids giving feedback. If feedback is useful, you should have already given it while you still hoped to salvage the relationship. At this point, you should be looking to sever the relationship as smoothly as possible.
It puts the control in their hands over the exact termination of the relationship. With notice or immediately is their decision. But it puts clear limits on their options, so you aren't stuck having the same conversation again in a week or two.
The suggestion that another tutor might do better is face saving for them. Even if untrue, it leaves them their dignity. And if they get the same problem with future tutors, then perhaps they will eventually adjust. Or give up on the idea of tutoring.
Another option is to suggest breaking off the relationship when the parent is being difficult. E.g.
I'm sorry, but I can't do that. If that's what you want, perhaps you should engage a different tutor.
The exact wording should be dependent on the situation.
That may be discourteous, but the context would have them being discourteous to you at the time. In those circumstances, there is no need for feedback, as it is obvious what the problem is.
Before you get to the point of ending the tutoring relationship, you should give feedback about unmotivated students. Appeal to the parents to see if they have suggestions to engage the student further. Presumably they know their child better than you do.
If you always give immediate feedback, then there isn't a festering concern on the relationship. So if you get to the point of terminating, there should be no need for more feedback. You've already said everything that was necessary.