older child; elder child
Refers to the older of two or more children, typically within a family. Often used when comparing siblings.
上の子が小学校に入学した。
My older child started elementary school.
上の子は下の子の面倒をよく見る。
The older child often takes care of the younger one.
Compound of 上 (うえ, 'above, upper') + の (possessive particle) + 子 (こ, 'child'), literally 'the child above'.