"A child is short."
Translation:아이는 키가 작습니다.
키 means height (of a person). 가 subject marker. When speaking of a person's height in Korean, you always pair 키가 + adjective. 키가 작합나다 / 키가 큽니다.
So, the literal translation for this is "As for the child, his/her height is small'?
Basically, though more literally the "his/her" part is not there but implied