Is 子 ever used to refer to a boy, or is it always used for girls? Is 悪い子 used only for girls?
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1This question probably comes from noticing how common '-ko' is as an ending for girls' names. – jkerian Aug 13 '12 at 23:16
Yes, 悪い子
can be used for boys.
子
means "child", and it can refer to girls or boys depending on the context.