I believe I have been able to reproduce the issue (both on 6.3 and early 6.4 build).  The scenario is essentially the same as the initial description; however, I am going to the specifics that I used.

Scenario:

- enable/sync
  - Red Hat Enterprise Linux 6 Server Kickstart x86_64 6.10
  - Red Hat Enterprise Linux 7 Server Kickstart x86_64 7.5

- create parent hostgroup:
  - Name - parent
  - Lifecycle Environment - Library
  - Content View - Default Organization View
  - Content Source - [pick the satellite server]
  - Operating System:
    - Architecture - x86_64
    - Operating System - RHEL Server 7.5
    - Media Selection - Synced Content
    - Synced Content - Red Hat Enterprise Linux 7 Server Kickstart x86_64 7.5

  - click Submit

- create child hostgroup
  - Parent - parent
  - Name - child
  - Operating System
    - Operating system - RedHat 6.10

    - ISSUES:
      - Media Selection & Media - both are disabled - grayed out

  - click Submit

Note:
- child hostgroup is created
- click on the child hostgroup to edit it
- click Operating System

- Observed that although I was not able to select a Media, the following is selected:
  - Media Selection - Synced Content
  - Synced Content - Red Hat Enterprise Linux 6 Server Kickstart x86_64 6.10

So, although I was not able to select it, the desired media was chosen based upon the Operating system.

The Satellite Team is attempting to provide an accurate backlog of bugzilla requests which we feel will be resolved in the next few releases. We do not believe this bugzilla will meet that criteria, and have plans to close it out in 1 month. This is not a reflection on the validity of the request, but a reflection of the many priorities for the product. If you have any concerns about this, feel free to contact Red Hat Technical Support or your account team. If we do not hear from you, we will close this bug out. Thank you.

Thank you for your interest in Satellite 6. We have evaluated this request, and while we recognize that it is a valid request, we do not expect this to be implemented in the product in the foreseeable future. This is due to other priorities for the product, and not a reflection on the request itself. We are therefore closing this out as WONTFIX. If you have any concerns about this, please do not reopen. Instead, feel free to contact Red Hat Technical Support. Thank you.