Remember that people could only just about grow enough food for themselves. This is called subsistence farming. They had very little surplus - sometimes none at all. Most people lived on the farms around the city, not in the city itself. Only the wealthiest had homes in the city as well as the country. Children would help herd goats and scare off birds, while men and women - and particularly slaves – would till the ground using oxen and a simple stick plough.
