Any helpful hints as to when NA referes to AT and when it referes to ON. Or is there no way to make this distinction?
Most of the time No(s), na(s) is the translation for "at,on,in the"
Yes, but the answer "The children are making sand castles at the beach" was not accepted. Should it have been?
I think it depends more on the English than the Portuguese. In this case, "on the beach" and "at the beach" have ever-so-slightly distinct meanings. But both should be accepted.
I would use "on" when I'm talking about the surface, and "at" for location.
You can swim "at" the beach, but not "on" it.