Translation:Arab women and children arrive at the city.
"Coming" is a movement to a place, which would warrant using the suffix -hoz in this case. But here you have -nál, so the movement already has taken place. They are not 'coming' anymore but they're now at their destination.
It's a bit a weird concept, I admit, but you can't transcribe everything literally into English.
But in that case, you need to supply "arrive" as one of the translations of "jönnek" This is similar to the sentences with "-hoz áll" -- the apparent mismatch of motion and stasis is a feature that conveys something in Hungarian that is difficult to translate into English.
The Hungarian sentence does not mean that the women and children are "arriving at" or "coming to" the city. I think that "arriving by the city" is a valid English phrase if their destination is near(by) the city. The Hungarian sentence says nothing about entering the city, that would be városba or városban, depending on whether the city is their destination (and they are outside of it), or their destination is within the city (and they are already inside).
- Arab women and children are arriving (or "coming to") near(by) the city.
I think this is why the author of the English translation used 'by.'
However, you're right about the meaning of jönnek, which indicates that their journey has not come to an end: megjönnek or érkeznek would be the translation of arriving. The weird thing is that the Hungarian sentence uses jönnek together with városnál instead of városhoz.
This could only mean, as I imagine, that their destination is not the city (not even closeby), they are just sort of "coming around" it: -nál indicates that they are already nearby.
- Arab women and children are coming around the city.
But this is a problem with a few of the sentences here on Duolingo. The English sentence is basically meaningless. If you say to me you are "arriving by the city" in English, I don't know what you're trying to say - it's not a natural sentence. Maybe that's because if you "arrive by" something in English it's more likely to be followed by a mode of transport - e.g. "I arrived by car". Arriving "near" the city would be a lot clearer. That said, when I asked a native speaker about this sentence they told me it doesn't sound natural in Hungarian either.
I think you might have interpreted the phrase wrong. It's the object "a városnál" - to the city, and the subject "arab nők és gyerekek" - Arab women and children, without a "the".
The way you phrased it sounds like you expect something about "the Arab city", which is not the case here.