Is 'kalian' = 'you all' rather than just 'you'? Since singular 'you' = 'Anda' or 'kamu'. My answer of 'You all are princes' was marked as wrong.
Kalian is a plural form of 'you'. It doesn't translate directly to 'you all', although that is our closest English equivalent, I guess. I don't think your answer is technically wrong, but it might be too colloquial/regional for duolingo, since 'you all' or 'y'all' is only spoken in some American regions. Most of the time, English doesn't differentiate between the plural and the singular 'you', which I would guess is why duolingo chooses to translate it this way.
There should be an apostrophe as such" princes' "if it is going to be male or "princesses" if it is going to be female since Kalian is plural form of you.