Display subpage Thumbnails with its excerpt

Hello There,

I’m absolutely a newb and have basic understanding on codes. I’ve been reading up a lot on the how to display excerpts with thumbnails for child pages..but have been able to get no where. I’ve seen codes being discussed on various other posts but have had no luck getting them to work. Frankly because i’m really not sure where i’m to use them. I’ve designed my site using the Minimatica Theme, and have made few minor tweaks in the Style.css to suit my needs (https://www.marshalz.in). I wish to use this display option as i’m going to have a lot of pages under a few parent pages. I want use the thumbnails and excerpt box/space to be hyperlinked to its source page. So when a user clicks it, he’s directed to that specific page.

I would request anyone who can help me get this sorted out.

Many thanks in advance.

Reuben