I think the "shall we" version would be more closely matched by "... üljünk?" Which is the imperative version of the verb.
But it is up to the context to pick the best version. We could also say:
"Odaülünk azokra a zöld székekre?" - this is kind of an invitation or suggestion to go and sit there. So, a good "shall we".
It is so frustrating to repeatedly give answers that should be accepted and then have them rejected. I used "onto" instead of "on" which is technically more accurate than the accepted English translation since it actually implies movement. I wish there were an option where a user could override Duolingo's evaluation as to whether a translation is good or not.