The "green plain" in the dream represents a better future for the boys.
Explanation:
The Chimney-sweeper by William Blake is a beautiful poem that tells the story of a child that is sold by his father after his mother passed away. The green plain is a representation of a dream in which Tom, speaker's friend, and the other chimney sweepers are freed by an angel; and after that they run to take a bath in a clear river to be so clean and white under the sun.
In this case, the green plain represents a place of fertility, spring, growth and freedom in a place where that children can run happily sharing with the other friends with no fear, and with a great expectation about the future.
