Gretchen, when the passed the plate, the President had no cash on him. Presidents of the United States never carry cash, credit cards or ID. He looked a little sheepish, and his father reached over and gave him a few bucks to toss into the plate. The Vice President did have some cash, which explains his smile, I think.
Presidents don't normally carry cash because they don't carry a wallet. But Dubya has carried it during campaign stops where he has paid for his own food in resturants. I have seen him pull cash out of his pocket. He probably didn't think about cash for this as it was his inaugural service.