warui ko 悪い子

In Japanese, warui ko 悪い子 means a "bad kid," in the sense of a misbehaving, naughty child. It can also refer to a "bad," Warui 悪い, adult or animal, who's like a naughty kid—doing mischief and stuff—because of how Ko 子 works.

The antonym would be ii ko いい子, "good child."

Usage

The most basic use of warui ko 悪い子 is to directly call a child bad:

warui ko da ne 悪い子だね
[You're] a bad kid, [aren't you?]

Often, warui ko isn't used to directly call a child bad, but to talk about "bad children" in general. For example, in fairy tales, etc. used to keep children from becoming bad children:

warui ko niwa Santa-san ga konai
悪い子にはサンタさんが来ない
For naughty children, Santa-san doesn't come.
- Santa-san サンタさん
  Santa Claus. (from Christmas.)
warui ko niwa Kuroi Santa ga kuru
悪い子には黒いサンタが来る
For naughty children, the Black Santa comes.
- Kuroi Santa 黒いサンタ
  Knecht Ruprecht, known in Japan as "Black Santa." (from Germanic folklore.)

Toward Adults

In anime, there aren't many young children, there are mostly teens, and some adults. But the word warui ko 悪い子 is used nonetheless.

When this happens, it has a nuance of "having done mischief" or "not being well-behaved." Obviously, this isn't used formally, it's used jokingly, or mockingly, or playfully, etc.

Manga: Gintama 銀魂 (Chapter 31)

Context: someone bombed a festival. Characters not happy.
matsuri wo jama suru warui ko wa...
祭りを邪魔する悪い子は･･･
The naughty kid [that bombed] the festival...
- jama suru 邪魔する
  To hinder. To be an obstacle to. (in this case, hindered the festival by bombing it.)
da~re~da~
だ～れ～だ～
Who~ [are~ you~]
- They're searching for culprit.
- matsuri wo jama suru warui ko wa dare da?
  祭りを邪魔する悪いこは誰だ？
  Who is the naughty kid [that] hindered the festival?

Related Vocabulary

For reference, some relative vocabulary.

A warui ko 悪い子 is normally a:

warui koto wo suru kodomo
悪いことをする子供
Child [who] does bad things.

Sometimes warui 悪い is part of another word:

seikaku warui ko 性格悪い子
A kid [that has a] bad personality.
sodachi warui ko 育ち悪い子
A kid [that had] a bad upbringing.
- sodatsu 育つ
  To raise [a child].

A few words that generally accompany warui ko:

oshioki お仕置き
Punishment. Spanking.
warui ko niwa oshioki ga hitsuyou
悪い子にはお仕置きが必要
For naught kids, spanking is necessary..
Bad children need punishment.

batsu 罰
Punishment. (general sense.)
batsu wo ataeru 罰を与える
To give punishment. To punish.
taibatsu 体罰
Corporal punishment.
- Spanking, hitting with something, kneeling on rice, etc.
- This is the sort you often hear that parents shouldn't do to their kids: that they should listen and talk instead.
rouka ni tataseru 廊下に立たせる
To be made stand in the corridor.
- One sort of taibatsu found in anime when students misbehave. (in real life this kind of thing isn't as common as in the past.)