Just happen to be watching, and rewound the beginning three times in order to figure out what happened. According to the police chief, Mr. Wentworth somehow ran over the mother and child, although he is not shown actually doing so - you never actually see them until you see the bodies on the side of the road.
It seemed more like they had been there for a while, frozen, but the dialogue says otherwise. At first I thought it was his wife but couldn't figured out who the other child was.
I'd always understood it as the mother and child had been stalled by the road and when Mr. Wentworth lost control of his car he ran into them.
Lily-32 is correct. A mother and her child were driving and their car stalled so they went walking for help and when Charles Wentworth lost control of his car because of the snow and ice he killed them. It was ruled as an accident and no charges were brought against him.
I hope that answers your question.