"They are asking for cookies."
Translation:Proszą o ciasteczka.
Well... I don't know what to say, because it simply is that "prosić o" is "to ask for" (as in 'to have a request', etc.). And it takes Accusative, which is identical to Nominative for not masculine-personal plural.
We have separate verbs for 'asking' as requesting and for asking questions.
The people that created the course chose to teach "ciasteczka". Then other people (we) came, and considered this to be a rather strange choice. "ciasteczka" is the diminutive form. It's like saying "little cookies". "ciastka" is the basic word. We can't change the sentences to say 'ciastka' as it hasn't been put in the database, we can only accept this answer - but we can put the answer we consider a better one in the hints. Not a perfect solution, but well, that's what we can do.
A similar situation, but also a bit different is with socks. The course teaches "skarpety", which to me is an augmentative and I associate it with big, warm winter socks. (I mean normal socks for your feet, although potentially also those that you hang on Christmas, although it's not a tradition here). To me, the basic word is "skarpetki". Well, different choices.
As for "cakes"... Well, some people claim that "cake" = "cookie", but I'm really not persuaded if it's worth accepting. Even if it can be used like that sometimes, mostly it's another thing, like a birthday cake. And that translates to "ciasto". Or "tort", actually the birthday one is rather "tort urodzinowy", but you get the picture.