How the get all the child menu's Node IDs? I've a very simple task to do in Drupal 8, but I've hard time to find the proper way to do that. 

I have a simple menu like this (all basic pages).


Company


Organization
Values
Terms

Services
Team


And when we go to Company page, I want to display all three sub-pages content in there.

How do I get the sub page node ID from the current menu? In Drupal 7, I'd go with something like menu tree data, and parse the URL to get the node ID; I'm sure we can do something similar with Drupal 8, but I can't find a way.
I think you're approach is just from the wrong end, the menu is just a list of links and not meant to control content.
What you want to do is:


Make "Company" as a views > page or a views blocks within etc.
Make a taxonomy reference for the sub pages or make them separate entity or content type,  in order to specify your sub pages in views.
Create view_mode like teaser to format their subpage display.