πάω σινεμά and πάω στο σινεμά are both used, it should not be marked wrong
We are still adding tnaslations like this one, since they were not initially added. It's been added, thank you.^.^
Why not κινηματογράφο which is used earlier.
"Εμείς θα πάμε στον κινηματογράφο" is the best translation. Are you sure that you used the right article (στον and not στο, since κινηματογράφος is masculine?)