"bull bar" means: A welded framework of metal bars at a vehicle's front, designed to protect it during collisions with animals, but often purchased as a fashi.... 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.