"Marmageddon" means: A national shortage of Marmite in New Zealand from early 2012 until late 2013, following damage in the 2011 earthquake to the Christchurch f.... 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.