page.php vs. index.php

Hello,

I have just created a sub-menu for one of my menu items. All of my menu items are pages that simply display posts of a specific category. The parent-level menu items seem to be displaying the category posts using index.php. However, the child-level menu items are using page.php to display the posts, and that’s creating a different look.

Is there any way to get the child-level menu pages to default to the index.php template?

The pages in question are:
https://www.boundingoveroursteps.com/reviews/ (index.php)
https://www.boundingoveroursteps.com/reviews/restaurant-reviews/ (page.php)

At the end of the day, I want the Restaurant Reviews page to display in the exact same way as its parent page, Reviews.

Thank you for your help!
Mindy : )