Vocabulary: Kids

Kid is an informal word to refer to a child.

I'm taking the kids (= my children) to the zoo this afternoon.

The word kid can also be used to refer to a young person:
college kids

These soldiers are just kids.

In American English kid can be used as an adjective: (of a brother or sister) younger

his kid sister

Kiddie is an informal word to refer to a small child

I carry my little boy in a kiddy seat on the back of my bike.

Kiddo (esp. Am E spoken) is a way of addressing someone you know, usually a young person.

Come on kido, let's go.

Kid's stuff (in Am E) something that is suitable only for children, because it is too simple, unsophisticated or boring.
The exam was kid's stuff compared to the one we had last year.