I'm not wholly sure what you mean by "UX examples" of a pattern where subsites are similar to a parent site. Isn't it just called consistency?
Subsites aren't really things your average user clicks through going "oh, look, they all look the same! How nice!". Only the owners do that. And user interface designers.
You could say it's brand consistency and that's what the user experiences so by way of having a consistent brand throughout, the UX is consistent. On the other hand, the consistent visual design is what the user actually sees, and the consequence of that is that the user interface is recognisable and therefore presumably more intuitive to use because you don't have to figure out the navigation each time you visit each subsite.
That said, I've always liked what 37signals does with its products' marketing sites. Although Backpack and Campfire differ from Basecamp and Highrise because the sites are in the process of being redesigned, the navigation bar and footer are consistent across all the sites, so I always know where to go for the basics (i.e. features, pricing, support, sign in, etc) and for links to general 37signals stuff and other products (the footer).
The takeaway for me here is that these are the two key elements that you want to design to be consistent. It feels in sync with the likely mental model of visitors: I might have multiple accounts with several of their products and therefore visit various product homepages when I want to sign in. If the sign in button is in the same place, it's easier for me to do that. Or I might have a Basecamp account and be considering signing up for Highrise; similarly, I know where to find the pricing page. And if I'm looking for more info about 37s or I just want to explore, I can go to the footer on any site. This makes sense, but doesn't force 37s to fill in the "content" of the site in the exact same way everywhere as this wouldn't necessarily add much. I know they're doing it anyway, but it doesn't feel like they need to if they get these navigation elements right.