How does one run a successful Ant farm?

My 4-year-old son got an ant farm for his birthday. We set it up and sent for some ants which came in the mail and looked fairly healthy. We fed and watered them regularly but they have been steadily dying off, In fact we have only one little trooper left, We have had the farm for about a month or less. What might we have done wrong? I don't think I put too much water in, and the ants seemed extremely active up until their death. They didn't die all at once. Any ideas? (And no, the boy was not allowed to handle the ants)