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

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.

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

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.