Am I the only person who thinks that there's something wrong with this story? First, foxes very rarely enter houses. Second, even urban foxes are very timid, so having to beat one to make it let go seems unlikely - it would leg it as soon as it heard a person. Lastly - seriously, the door was open in this weather? Were they trying to kill the baby with hypothermia?
Two much more likely scenarios:
1) It was a rat, not a fox. They do come indoors, they do attack people, and they do hang on when struck. And some can be very big. The parents were just mistaken. Or...
2) It was a family dog, but they feared that the animal would be put down if this came out, so the parents made up this story.