"Are you little girls?"
Translation:Vy jste malé holky?
Adjectives in Czech are declined. They have have grammatical genders and grammatical cases and also numbers. And they change forms according to the current gender, number and case. You have to remember all those individual forms and determine the gender and the case in the sentence.
Here "holky" is feminine nominative plural, so the adjective is also feminine nominative plural. And the form for feminine nominative plural is "malé".
In the Tips and notes https://www.duolingo.com/skill/cs/To-be-plural/tips-and-notes or in one of the recommended textbooks.
Both in Czech and in English, questions can be declarative (in the affirmative order). https://www.thoughtco.com/what-is-a-declarative-question-1690372