"Those sandwiches, they are not eating them."
Translation:Esos sándwiches, ellos no los comen.
As in English, the present progressive form in Spanish is formed with a participle, here "comiendo." And only forms of "estar" are used (which makes sense, "estar" is generally for states that are changeable, and they're not always eating). So, all told, the second part of your sentence should be "ellos no los están comiendo" or "ellos no están comiéndolos," but it may well be more common to use forms of "comerse" here, as ProfesorAntonnio did.
To your first point, whoops, sorry, my bad; missed that rather obvious point.
To your second, my point was that Iris didn't write "ellos no los están comiendo", but instead had the answer without the gerund, ergo the gerund was not the problem. "Son" was.
In saying that, given that Duolingo allows "Esos sándwiches, ellos no los comen", but using the participle/gerund seems like a better translation, what would you suggest?
Yes, if Iris had just left out the "son" the answer would have been right, but I assume I was correct to infer that she wasn't trying to just use the ordinary present simple conjugation but rather fashion the Spanish present progressive but didn't know / recall how to do so.
I'll have to defer to others as to suggestions. It seems there are at least a half-dozen versions for that last portion of the sentence, all of which I presume should be accepted, but which you'd hear most (or at all) probably depends a lot on where you are.