"noughties" means: The decade consisting of the last year of the previous century and the first nine years of the next: from 1900 to 1909, 2000 to 2009, etc.. This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.