"The child saw themself in the mirror."
Translation:아이가 거울 속에 자신을 봤어요.
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.