I have found no solution or even a mention of this problem on the web. This makes adding parent hairs in particle edit useless for interpolated children. I think it is a bug. I’ll do the eyelashes and eyebrows without particle->mirror but it would be a time saver if it worked.
https://blenderartists.org/t/guide-parent-hair-created-in-particle-edit-ignored-for-interpolated-children-how-to-fix/1248891
No solution for the coarse render of hair with cycles either. Looks ok if not close but this should be fixed.