Recommendations for Multisite setup?

This is my first multisite in WP (I’m excited!) but I was just wondering if anyone could help me with some recommendations for how to structure it.

Here is what I need:

There is an “umbrella” site, that has its own pages and nav etc. Its index.php is a landing page to jump to the two child sites, which also have their own pages and nav. They all have identical templates save for some colors and background images here and there.

I’d like to use a subfolder setup because that makes the most sense to me, but my question is, what would be the best way to set it up so the parent site is the “base” website (i’m thinking of the url as http://www.site.com) and the two child sites within it (www.site.com/childsite1/)? From what it looks like in multisite, I’d have to create 3 sites all within their own subfolders?

Also what is the best practice for a url structure, when the base site is going to have its own heirarchical content (www.site.com/aboutus/), but will also have child sites represented by the same folder structure (www.site.com/childsite1/)? Just looking for opinions on the best way to organize this.

Thanks for any help!!