"The girls' children"
अनुवाद:लड़कियों के बच्चे
Good question! You are right, it doesn't really make sense - girls cannot have children, but many sentences on Duolingo don't make sense (there are sentences like "I am an apple" and "The bear drinks my beer" on other course) - it is more about being able to understand the grammar and being able to correctly translate.
Having said that this sentence is not completely impossible. I know many parents would (almost jokingly) call dollys children and say things like "look at those girls' children, aren't they all playing nicely" if they saw a group of girls playing with dolls.