I have selected "is Child-issue" in our import system (CSV), and it consistently comes through with a link type of Parent.
I even tried selecting Parent (just in case my thinking was twisted) and of course, it came through as Parent again.
I have been able to do "Relates" link types quite successfully, but not this Child relationship (and they are not task/subtasks)
I see comments on Inward and Outward - but it isn't a blocking issue...
When I actually Create a link, on an issue - I can create a "is Child-issue"