I think there is no way to sort it based on the user added way with relationships. But you could achieve this with creating another collection to store the exercises of that workout and sorting that list as Created Date- Oldest to Newest.
Made a quick clonable app :
And made a video too :
Jason ( @jasondupree ) made a wonderful Workout app and that app has this sorting and I think he also did the same way or maybe a different way!