子 (ko) means “child” | MotiTan Japanese Dictionary

More examples

あの子は私の友達です。a no ko wa watashi no tomo dachi de su

That child is my friend.

先生がその子を呼びました。sen sei ga so no ko o yo bi ma shi ta

The teacher called that child.

Refers to a young human viewed in relation to a parent or family.

子供kodomo: A young human who has not reached adulthood.
お子さんokosan: A polite term for another person's child.

親から生まれた人で、成長の途中にある者。oya ka ra u ma re ta hito de sei chō no to chū ni a ru mono

子 (ko, "child") is a native Japanese noun with an old basic sense of a child or offspring, while its deeper origin is uncertain.

Japanese often uses 子 (ko) in casual references to a child when the child is already clear from the context.