There are always agreements between the API designers and client designers, because the code isn't intelligent, so it doesn't know what 'parent' means. If the name of that changes, then the client would have to change. What can be different is the url structure to get the 'parent'. Maybe there's a new token that has to be there, or the naming of something has been changed, or whatever. Those changes can be made without breaking the clients. Putting this in the responses means two things:
1) Small changes don't break everything (pagination is a great one, its a /pages/1 one day then ?page=1, then ?offset=25, then ?pageSize is optional, then it's required, etc.).
2) Fewer assumptions baked into the client. Getting the tweets for a user you've loaded would be nicer as load(user.tweets) than load("/tweets/"+user.id)
watch these two very good talks on this topic:
I can see how returning hypermedia adds yet another layer of abstraction (and potentially plenty more round trips!). I'm just unsure how it helps. I don't understand how actual client code (besides a browser) can deal with arbitrary hypermedia. I'm cautious when I don't understand why people are hyped up about something, but I've yet to see any "real life" examples that demonstrate real benefits of this approach.