CAA15, first of all, the יי doesn’t replace the ים because double yod is first person plural possessive and ים merely marks a plural. Putting aside biblical Hebrew, in the Tips section, we learned that susi = my horse and susai = my horses. So the יי marks the plural possessive for something of mine.
Next, if you said אתכם, you would be changing the sentence from you all feminine to you all masculine. Lastly, if you say ילדותי (yalduti?), you are changing the sentence from my beautiful girls to my beautiful girl.
However, if you write yaldotai with niqqud, then a single yod at the end can mean my girls. See Pealim.
I love Duolingo. Hey, free - I guess some problems are expected. Sound - looks like this has been an eternal problem. I'm a web app programmer - browsers were really never meant to do all the things people want them to do. So, I understand the limitations. But I got this one wrong because I made the mistake thinking I heard the whole sentence - I didn't. :( Get use to it. Unless the developers come up with a more successful way to do sound - keep your eyes and ears open when the page loads and click on the sound button more than once - I didn't :-)