Thank you Mika. I had a look at it and it is exactly what I had in mind.
I saw this note to it:
This plugin hasn’t been updated in over 2 years. It may no longer be maintained or supported and may have compatibility issues when used with more recent versions of WordPress.
I imagine that this kind of wp setup is appealing to other, so I wonder why it is not updated. I fear a little bit that it becomes obsolete. Are there other common solutions for setting up such networks?
Thanks Noahjonah!
I had the CSS all mixed up. So I copied it completly to the child them and installed a default style.css into the parent theme.
Not very elegant, but it works.
