Yes, qmr, I should have made more of the playful elaboration aspect I only even mentioned as an afterthought. There is certainly a strong association with children for the -day and -daisy forms. It's all part of the lifting 'up/oop', isn't it? Hence the multiplying elaborations upsy-daisy/oopsy-daisy/whoopsy-daisy, and as I said above, I have been aware of all the -daisy forms supplanting the -day forms in my own lifetime.
But I have also been aware of distinctions being made between the ups/oops forms and the whoops forms for accidents (hence the 'whoopsie'). OED says 'whoops' may a natural exclamation which has become confused with ups/oops.
We agree that it never had anything to do with swearing (can't think what gave Bob that idea), but not that that the ending might just as easily have been 'dandy', 'dinky', 'doozy', or any other similarly rhythmic sound. No doubt I did give too much detail about the etymology, but my point was that the 'day' was already well established in alack-a-day/lack-a-day/lackaday (alas the day, woe is the day) > �lackadaisy�, from which we get 'lackadaisical', and the analogy and the elaboration were from that.