"The girls are wearing hats."
Translation:De meisjes dragen hoeden.
Your translation is absolutely right. If Duo wants to hear "De meisjes dragen hoeden" the question should have been: "The girls wear hats." Nevertheless, you'll never hear a dutchie speak out 'De meisjes zijn hoeden aan het dragen'. They will shorten it to ... 'De meisjes dragen hoeden'. That's the problem Duo is facing: The same sentence in dutch can mean two states of ... An actual state and a general one: 'The girls are wearing' will mean in dutch that the girl are wearing hats at the same moment of speaking; 'The girls wear' means the same but without the necessity of actually wearing it now. It depends on the context (in terms of place and time) what is meant to be said.
And another thingie: If someone will speak out 'De meisjes dragen hoeden' a dutchie will know that you are either an infant or a foreigner. The dutch will say: 'De meisjes dragen een hoed' wich doesn't mean that the girls have been stewed and blendered under only one hat. ;-) Don't try to find any logic explanation for it for there isn't one. That's just the way the dutch speak ...
Plural is always 'de', and for the singular thingies ... There are them theoretical rules wich only place the same problem on an other table (masculin/feminine): in practice you've just got to remember wich word is a 'de' or a 'het'-word. Oh! Another practical rule: All words ending with 'je' are 'het'-words: Het huisje (the little house), het jongetje (the little boy), het woordje (the little word).