Translation:The healthy boy with the girl counts olives.
I wrote "The healthy boy counts olives with the girl" and it said "The healthy boy with the girl counts olives." would be "another correct solution. If not grammatically incorrect, the latter certainly sounds very clunky and should not be the intended solution.
I wanted to report this, but there is no checkbox for it and "something else is wrong" doesn't give me a text field.
Duolingo never said it was the intended solution. It's just letting you know that it's another accepted answer.
I personally believe that it's better for Duolingo to be a little too permissive when accepting answers than to be too strict.
Believe it or not, some sentences that might sound too clunky to (y)our ears may sound perfectly normal to other native English speakers. (I've been in that position before, and insisting that their way of speaking English is wrong doesn't help anyone, and makes me look like I've got control issues.)
Can't you use the plural numerant for puer cum puella are two? If the verb must coordinate with the boy because cum subordinates the girl, in the sentence, to him, and verbs must fortunate with the boy...
If it was puer et puella, with et instead of cum, then it would plural?