Sorry, I can see how I sounded confusing in my reply. 

You can, and perhaps should have some kind relationship between Parent and Child spaces besides simple reference. The thing is that can't see one commonly acceptable approach that would make sense in every scenario. So, the Spaces module is much like a prototype module, which we use to create a modified version for each specific idea. 

Here's a hypothetical scenario: 

- A site for US universities students. Each member must have an email in their uni domain. Spaces for each uni are pre-created. Members are auto-assigned to their university space according to their email domain. 

- Parent spaces for each State in US are created and universities are assigned as child-spaces into their respective states.

- The second level of children spaces are created for, say, faculties in each uni.

- The task is to make sure that students socialise, collaborate and share with a good level of relevance, without too much noise.  

....

We would have to configure spaces somewhat like this:

- Auto-assign profiles as members of specific university spaces according to their email domain.

- Make faculty-level spaces invitation-only or "approval required", so that admins of the spaces approve members (because we can't programmatically know to which faculty a student belongs). 

- Allow Students (membership level) to post to Faculties only - the most ambient level of spaces.

- Allow Staff (membership level) to post to Faculties and University spaces. This will ensure that only high-level announcements go into those spaces and the potential for spam or rogue/unsafe content exposure is contained.

- Push "popular" posts upstream with a certain daily limit (say, 3 most liked posts from each faculty go into University and 3 most liked posts from University goes into State) - this will control noise and curate the best content for more exposed spaces.

- Set students to auto-follow their University and Faculty spaces, and recommend to follow State. Perhaps, allow them to follow other spaces if they want to.

- All of the above will populate their feeds. Naturally, they may be choosing to also follow specific students or teachers or groups if they want to. 

__________

See, this is just one potential application scenario. Your idea may be completely different, but you always need to look for ways to make feeds populate with the most relevant content with digestible pace. Say, if you auto-assign a Student to a State and push all posts from all State subspaces upstream, then the feed of the Student will be full of content from various universities, different faculties, etc, which may quickly become unusable and Facebook-like. 

And when I say Facebook-like I do mean it in a bad way. See, Facebook curates content in their so-called "smart feeds"  based on two main factors - what makes more money and what makes you use Facebook more. In most cases, it boils down to observing your tendencies and rehashing them - if you like a "right-wing" posts, your feed will be predominantly populated with "right-wing" content, and vice versa. As a result, everyone is living in their own version of a virtual comfort zone - an information bubble that we first trigger by our choices and then Facebook algorithms reinforce manyfold. This is a horrible, disgusting, appalling effect, which should be disrupted - it inevitably feeds division between people. (sorry for digressing - it's a topic that's close to my heart, so to speak).