Exactly. I was given the sentence "Die Hüte von ___ Jungen sind groß." and asked to fill in the article, so it would be either dem if Jungen is singular, or den if Jungen is plural. The problem being that the rest of the sentence is identical whether it's plural or singular and the article is the only thing denoting plurality or lack thereof, so unless you already knew the sentence that they're using (which I didn't), you could only 50/50 guess at what the correct article is.
"von" and dative is able to be used here instead of genitive, same as in english you can say "the boy's hats are big" or "the hats of the boys are big" (so the translation implies we should be using genitive which I think would be "Die Hüte der Jungen sind groß") not sure if that one is accepted on this sentence as I haven't tried it.