"The child eats between the man and the woman."
Translation:아이가 남자와 여자의 사이에서 먹어요.
사이에서 translates to between, but 사이 alone means "space" so sometimes Koreans add 의 to the nouns that the other thing is between. It's not necessary, but it is used here and there. The meaning doesn't change as far as I know. If that's the case, they should accept the sentence with or without the -의.
남자와 여자의 사이에서 아이가 먹어요 was marked incorrect because of the order. Does it sound unnatural or weird of I say it that way?