Emergency services said the wintry conditions were thought to be a contributing factor.
Neighbour Victoria Bamber said Ioan came running down the country lane shouting for help to free his father.
She told reporters: “He came banging on the door saying ’There’s been an accident, Daddy is trapped’.
“When I got to the scene, I could only see the silver 4x4 upside down in the water and Ruth was desperately trying to find David.”
Mrs Bamber said she and others waded into the freezing cold water to try to free Mr Cox, and smashed the window with a large rock so the central locking failed and the car doors would open.
It is thought Tess escaped through the rear window which had smashed.
A police officer who arrived on the scene also waded into the water to help but, despite the efforts of emergency services personnel, Mr Cox could not be saved.
Mrs Cox and the two children were unhurt in the accident, police said.