Thanks as well for the screenshots as I was able to identify the exact issue. Given that you are adding the child post view to a content template and not another view, you will need to select the "The post where this view is shown" for the parent filter.
If you were adding the child view to a view that is listing the parent you will select the option "The current post in the Loop".
So edit your child view and change the parent filter option from "The current post in the Loop" to "The post where this view is shown" and this should resolve the issue for you.