Currently, the icons are defined by the page WorkspaceManager.Style and are located in the main wiki. Workspaces are aware of this and are explicitly importing the page from the main wiki as SSX so that the menu looks ok.
A regular subwiki does not know about this and does not use the SSX, thus not displaying the menu item icons.
The workspace application probably needs to use colibri.css to provide its icons so that they are visible on all the wikis.
Workaround for subwikis:
Add a page in the XWiki space of the subwiki and add a StyleSheetExtension object in it. In the content of the SSX object, add these lines:
Set the SSX to be used always on the entire wiki and that the content is parsed. Save and hard refresh (CTRL+F5). You should have the icons too now.