"You are not a child."
Translation:Nie jesteś dzieckiem.
Out of interest, I tried 'jesteście' instead of the informal. It was classed as a wrong answer. Does this mean, if talking to a royal child, for example, the informal jesteś would be used?
no. but "jesteście" is informal too, (adressing more than one person),
If you want to be formal, you use different structure, based on third person. Usually that would be "Pan/Pani/Państwo", but with royal child perhaps "książę"=prince , so "Książę jest dzieckiem".