Update child sites with parent attributed posts.

If I have a site:
https://example.com (alias: Parent)

And I want to make “child” sites for it:
https://example.com/colorado-springs (alias: Child)
https://example.com/boise
https://example.com/huntsville
https://example.com/austin

On Main, I want to post three posts:
ParentPost 1 (“/blog/parentpost-1”)
ParentPost 2 (“/blog/parentpost-2”)
ParentPost 3 (“/blog/parentpost-3”)

On Child, I would like two Child specific posts as well as the three posts
from my main site:
ChildPost 1 (“/Child/blog/childpost-1”)
ChildPost 2 (“/Child/blog/childpost-2”)
ParentPost 1 (“/Child/blog/parentpost-1”)
ParentPost 2 (“/Child/blog/parentpost-2”)
ParentPost 3 (“/Child/blog/parentpost-3”)

How would I automatically update Child’s site with Parent’s posts as I publish them? So when I post something in the Child site it will only be on the child site, but when I post something on the Parent site it will update all children.

[Bump deleted: https://codex.www.ads-software.com/Forum_Welcome#No_Bumping%5D