"witchety grub" means: The large, white, wood-eating larvae of several moths and beetles, especially the species , traditionally eaten by Aboriginals in the Austra.... 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.