"trolley" means: A trolley pole; a single-pole device for collecting electrical current from an overhead electrical line, normally for a tram/streetcar or a.... 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.