what is the more informal way to say this
Sweetie (referring to a child), stop eating lots of sweets or you get tooth decay. ...

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"... or you'll get cavities."  (carries, in British English)

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You can say "Sweetie, stop eating lots of sweets or your teeth will rot."

"teeth will rot" is a pretty informal way of saying you'll get tooth decay.

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I'm American, I would say "Sweetie, stop eating so much candy... you'll rot your teeth!"

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I would like to know is there any way to say 'tooth decay' in more informal style. I am sorry to forgot mentioning that in my question.