You don't know if they write one or several letters in the Norwegian sentence. So both ought to be accepted. However, since several grandchildren (barnebarna) are writing, you would think they each write one letter. Barnebarnet skriver et brev, barnebarna skriver (flere) brev.
Did DL really accept 'The grandCHILDREN WRITES a letter'? The grandchildren (barna) is plural so 'writes' is not correct.
'However, since several grandchildren (barnebarna) are writing, you would think they each write one letter'. Yes, that's what I thought as well and that was my answer the first time that I did the exercise.
I must confess that I can't remember if I wrote 'writes a letter', but you are correct, that would not be a proper sentence. So I think I wrote: The grandchildren write a letter. But that still doesn't make it clear how to know if they wrote 1 or several letters if you look at the sentence. I think that 'barnebarna skriver (flere) brev' as you said would make more sense.