What does the little boy in the squatter’s camp mean when he says: "When they need us they call us migrants, and when we've picked their crop, we're bums and we got to get out." What does that tell you about how the Dust Bowl migrants are viewed by others?

Dust bowl migrants were treated with impermanence and like bonded labor.

Explanation:

During the dust bowl many people from Texas had to migrate from the part of the country to other parts in search of work usually as labors for farm and were in turn exploited in a failing economy that could not keep up with the constant need for work of the growing population.

As the passage says the workers were treated as replaceable while they were called as someone who did not come from the part of the country itself at the same time being treated extremely bad by the runners of the farm.