The younger child in a family — used to refer to a sibling who is younger (contrast with the older child).
うちの下の子はまだ幼稚園に通っている。
Our younger child is still attending kindergarten.
下の子の面倒を見ているうちに時間が過ぎてしまった。
Time passed while I was looking after the younger child.
兄は大学生で、下の子は高校生だ。
My older brother is a university student, and the younger child is a high school student.