"Estuary English" means: A variety of English accent, spreading out from London into the area of the Thames estuary, containing features of received pronunciation, C.... 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.