You're right. It should be "these daughters".
diese Tochter = this daughter
diese Töchter = these daughters
That's not correct. Usually, there's no distinction between "this" and "that" or "these" and "those". The use of "jene" is very limited, especially in spoken German. If you want to be absolutely clear, you could say something along the lines of "diese hier" or "diese dort drüben".
Although true that "jene" is practically never used in colloquial German, to say that "this" means "dies" and "those" means "jene" is perfectly correct.
I mean, duolingo is not giving a course in colloquial German here. This is about learning the language, any implied meanings or cultural usage notes would probably overload the student with information and confuse him even more. Though this comment section probably serves as such and the curious student who wants to know more can sometimes find out additional facts about usages.
Heck, half the sentences duolingo provides sound so far out that you find yourself hard-pressed to come up with a situation in which they'd actually be said.