"thumbtack" means: A small nail-like tack with a slightly rounded head that can be pressed into place with light pressure from the thumb; used for hanging ligh.... 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.