Matthew was talking to people that would have been familiar with the Scriptures--what we call the Old Testament. So, they would have known that God provides food for people in the wilderness (See Exodus 16:13-35; Numbers 11:7-9, 31-32) and that God can feed a lot of people with only a small food supply (See 2 Kings 4:42-44). [Thanks to Allen & Williamson, Preaching the Gospels without Blaming the Jews.]
Of course, Jesus' disciples would have known those texts. Why did they think that they were incapable of feeding the crowds, that the only solution for hungry folks is to find their own food?
Well, we've read the same texts, and we've read Matthew's Gospel, as well. How much have things changed? Look at this article from the Wall Street Journal.