Because they are refering to both girls and boys or boys in general when they end with the o cansados if they use cansadas they will be refering to a group of girls and since ustedes was in there it means that there are refering to both girls and boys ustedes means you all
This discussion seems to refer to translating from Spanish to English or writing what you hear. In the first case, it would not be a translation, in the second, there is only one correct solution (which uses the male form).
Apparently, there are cases where the same discussion is linked to both translation directions. If this is the case here, your translation should be accepted.
Why was "Are they tired today" counted wrong? When I saw "están", I knew the sentence context was "plural" so I hovered over the hints and saw "you all" and "they". I do not like to use "y'all" or even "you all" so I entered "they"... I reported that my answer should be an accepted translation...
"Y'all" is a regionalism and so should be recognized as a word that some people use. I, myself, prefer to use "all of you" when translating "ustedes," even though the pronoun "you" is both singular and plural, as well as being a subject pronoun and an object pronoun. I use "all of you" in translation because it is less objectionable to native Spanish speakers, who have a hard time remembering that the English pronoun "you" does not need clarification.