Luke If the child page doesn't have a template (within the theme). The child looks if the parent page has a template. If so, use that template otherwise traverse the template hierarchy further.
Link to current Template Hierarchy: https://developer.wordpress.org/themes/basics/template-hierarchy/#single-page
Current template hierarchy concerning single pages:
- custom template file.
- page-{slug}
- page-{id}
- page.php
- singular.php
- index.php
Suggested template hierarchy:
- custom template file.
- page-{slug}
- page-{id}
- page-{parent-slug}
- page-{parent-id}
- page.php
- singular.php
- index.php
