younger child; youngest child
Refers to the younger or youngest child in a family. Often used in contrast with 上の子 (older child).
The younger child is still small, so they require a lot of attention.
At our house, the older child and the younger child play together nicely.
Compound of 下 (した, 'below') + の (possessive particle) + 子 (こ, 'child'). Literally 'the child below', referring to a younger sibling.