While that's true, if someone says to translate something to English and it's "Ses enfants", then without knowing whether you're talking about the mother or father, you would have to translate it as "His or her children", which is a bit unwieldy, and normally in English you would surely just say "Their children" which can also apply to just a single owner (I'm not sure if 'leur' in French can also mean just one person).
I am aware of this phenomenon, but you have to get rid of your English reflexes: "leur, leurs" are about more that one owner and translate to "their" and vice versa.
You cannot translate "son, sa, ses" to "their", never, and you don't have to: Duo accepts "his" or "her" or "its" to translate them, so up to you to pick the one that is meant.
Is there a way to express gender neutrality in possesives? I've gotten so used to saying "their" in place of gendered pronouns in English due to gender-queer sensitivities that it makes me wonder how to express this in French. I did it out of habit here and was marked incorrect of course.
No there isn't and there can be, since "son, sa, ses" agree with the object possessed and don't give any indication of the owner's gender.
On this course, everything you read and write must back-translate to the original sentence, and "their children" would translate to "leurs enfants", which can only mean that there are two or more owners.
These could have been my sister-in-law's children, writing to thank me for a birthday present, which she always made them do. My children often said they would do so, but forgot. My sister-in-law never failed to point this out until she, like me, lost control of the children we were responsible for.