To be able to define the tone of a poem, one must first understand what tone is.
Tone, therefore, is the attitude the author has towards the subject of the text and the reader's of the text. In regard to poetry, typically, one simply examines the attitude of the poet towards the subject depicted in the poem.
In Shel Silverstein's poem Where the Sidewalk Ends, the tone of the poem encompasses Silverstein's feelings about life and the choices one makes in life. The tone is depicted in the poem in one way: Silverstein wants readers to simply follow the lines in life.
Yes we'll walk with a walk that is measured and slow,
And we'll go where the chalk-white arrows go.
Here, what Silverstein is saying is that people must simply slow down and follow the arrows which point them in the direction which allows them to "leave this place where the smoke blows black and the dark street winds and bends."
There is one problem with this though. Sliverstein seems to go against the old adage of taking the path less traveled. Instead, he wants people to accept the fact that life's paths are drawn out for a reason and by following them people are able to stay away from corruption.
Therefore, the tone Silverstein provides for the poem is one of hope and adherence to the "rules."