Is there another method to allow access to grade only their own students' grades?
Thanks in advance!
I cannot think of any solution
I think that there has been some discussion about using separate groups for creating course sections and allowing section instructors access only to grades in the respective section. What you are describing looks like a similar scenario. Don't remember if there was a definite solution. The discussion should be either down this forum or down the groups forum.
Yes, separate groups should do this.
Enrol the parents in the course (perhaps using a custom role based on non-editing teacher, but without the access all groups capability).
Create groups in the course, so that each child and their parents are in one group.
Set all the graded activities to use separate groups mode.