There isn't currently a way to have a document that has other documents as children. In Clearspace, all documents are essentially peers, although the spaces/communities that contain them are hierarchical.
I faced a similar need when I first started putting developer documentation into Jivespace months ago. I knew that the flat "hierarchy" would provide no structural cues to help people understand the content in terms of functional groups (plugins as opposed to themes, for example). Tags seem to offer a solution, but you can't see a list of all the content sorted or grouped by tag; instead, you can see the content for one tag, then go back and see the content for another tag, and so on. Even so, you can't have content displayed in a particular order -- the order is chronological.
What I ended up doing was using the HTML widget to display a hierarchical list of links to each document in that community. Personally, I'm a believer in the smaller, more "atomic" way of storing the content, preferring to lean on an organizational layer or device (like that TOC) to provide ways into the content.
I've yet to upgrade the HTML-widget-as-TOC to the 2.0 version of Clearspace that Jivespace now uses. Soon, I hope.