"You are responsible for your son's mistake."
Translation:Du er ansvarlig for din søns fejl.
What is wrong with Du er ansvarlig for jeres søns fejl? Imagine you are talking to the son's father (but the son has also a mother). So "jeres" refers to both the parents, while "du" only addresses the father, which means that the mother is not responsible: You (the father) are responsible for your (the parents') son's mistake.
I would say that it is also correct. The sentence is posed in English, and could mean one of four translations.
"You are responsible for your son's mistake" :
"Du er ansvarlig for din søns fejl."
"Du er ansvarlig for jeres søns fejl."
"I er ansvarlige for jeres søns fejl."
The fourth translation would probably be less frequent, but nevertheless still valid. Think of a brother and sister who are jointly bringing up the son of the brother.
- "I er ansvarlige for din søns fejl."