"The child saw themself in the mirror."
Translation:아이가 거울 속에 자신을 봤어요.
It is right to interpret "in the mirror" as "거울 속에" However korean do not say like it. The verb "보다" which here "봤어요" is not normally with preprosition "에". Many Koreans also make this mistake. However the right one is, "거울 속의" which can write with the boy here
The English is also problematic. It would be much more more common (and probably more correct) to say The child saw himself / herself/ itself in the mirror. As I have posted previously, it would be nice to have a "central location" for calling this kind of issue to the attention of the developers. I suggest https://www.duolingo.com/comment/24887752 until a better alternative is provided.