"perp walk" means: The intentional public display before news cameras of a person in police custody, especially someone famous or notorious, for the purpose of.... 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.