Feeding baby corn snakes?

About a week ago I bought my sons two baby corn snakes and today was their day to eat. I went down to the pet store we bought them from and got two pinkies, but the ones the guy gave me are HUGE!!! I think they are baby rats or something and really really fat. I told the guy over and over that they were baby corn snakes and he said they would work. I got them home and one of the snakes acts like its not interested at all. The other attacked the pinkie and is trying to eat it but seems to be having a really hard time of eating it. He has been working on it for about 25 min and hasn't made much if any progress. Does this seem right? How long does it take a snake to eat it's pray?