"아이에게 무엇을 가르칠 것이에요?"
Translation:What are you going to teach the children?
In Korean, you don't need to always explicitly state plurals every time like you do in English. 아이 or 아이들 can both be translated to children, while only 아이 can be translated to child
But in that case you never have 100% chance of answering correctly. As Duolingo seeks perfectionism and literal translation it should have been child. Otherwise its just frustrating.
You can always flag up your submitted answer and say it should also be accepted as a translation. I disagree with you though; Duolingo shouldn't use unnatural terms just for accuracy. People are using the app to improve their language skills, after all.