Grinny

Grinny deals with a normal family in middle England whose lives change when they are visited by aliens. The alien takes the form of an apparently benign elderly relative who can play mind tricks on the adults in the family but not on the children. This Granny-esque figure is eventually caught out by the children's ingenuity (they realise she is no normal human being when she expects what they describe to her as a "cast-iron conker" to be made of cast iron).