"Radium Girl" means: A female factory worker who contracted radiation poisoning from luminous paint (containing radium) used in watch dials in the early 20th cen.... 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.