The angel said that if it were not for the Atonement, it would have meant that even innocent little children would have been damned and lost forever.
Of course, since the Father is all powerful, why couldn't he just go ahead and save us on his own? As we get into the Book of Alma we discover that if he had tried to go forward without the Atonement he would have actually "ceased to be God." Later on we will then discover why this is so.