Create parent-child relationships for Cases

To set up a parent-child relationship for cases in Classic:

To add 'Parent Case' field to the Page Layout:

To set Field Level Security for 'Parent Case' field:

To add the 'Related Cases' related list on the Page Layouts:

In Lightning Experience:

1. Steps:  Click setup| objects and fields| object manager| click on cases| 
2. Click on Edit next to page layout for which you wish to enable 'Parent Case". 
3. From Case (Support) Layout select Fields drag Parent Case and move it to your page layout.
4. Click Save.

To set Field Level Security for 'Parent Case' field:

1. Go to Setup | object manager| case| Fields & Relationships | 
2. Select the Parent Case field | click View Field Accessibility.
3. Click the Field Access for each Profile and select the Visible checkbox.
4. Click Save.

To add the 'Related Cases' related list on the Page Layouts:

1. Go to Setup | object manager| case| Fields & Relationships | 
2. Click Edit next to page layout for which you wish to enable 'Related Cases' list.
3. From Case (Support) Layout select Related list. 
4. Drag and Drop Related Cases into the Related List section of the Page Layout.
5. Click Save.