Meaning: This sentence describes the depth of the snow, indicating that it was significant and covered a large area.
Choice of Words: The word "deep" describes something that extends far down or inward. "Knees" are the joints between the thigh and the lower leg. When you put these words together, you get a picture of the snow being so deep that it reaches up to the children's knees, emphasizing the amount of snow and the wintery conditions.