"They are not visiting their parents."
Translation:Oni nie odwiedzają ich rodziców.
Oni nie odwiedzają ich rodziców. - First 'they' is a different group of people than the second' their'. -> (As) are not visiting (Bs)' parents
Yup, and "(As) are not visiting (As)' parents" would be "Oni nie odwiedzają swoich rodziców".
"oni" = there is at least one man among 'them'.
"one" = there are no men among 'them'.
Is the 'ich' necessary here? I wonder because it wasn't so in a previous question in this lesson.
It shouldn't be necessary. Also, "swoich" is a lot more probable than "ich". But it hasn't been introduced yet at this point in this course.
"swoich" would mean that it's about their own parents.
"ich" is... well, about some other "their" parents.
Added the option to not use any possessive, it's natural and it will be understood that it's about their own parents.