You can ask the child
They, laughing, award each other the most indecent words and expressions and think that it very witty and dared.
You can ask the child to stop it immediately or it is possible to allow to it to have fun time, and then to stop it.
Still shortly before children learn that mean these words, they already know that these words are indecent.
But anything human is not alien to children, they repeat these words to show that they already adult and not to be afraid to be bad.
Usually parents are shocked, having heard curses from lips of the of the innocent baby.
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