Format as Symbol
You don't have to look far to find symbolism in "The Ballad of Birmingham." The very format of the poem itself is symbolic. Broadside ballads, of which this poem is one, have often been used to describe harrowing events of the times. This format becomes the first clue that the poem will likely not end well. Note that broadside ballads -- a printed story in a song -- were often considered to be a form of journalism. Just as you wouldn't pick up a newspaper and expect to read an uneventful story of a child going to church, you can't expect a non-story in ballad format.
Questioner and Respondent
Within this poem, there is a format of "innocent questioner" and "wiser respondent," a setup that places the child in the role of a "conventional innocent," notes the Modern American Poetry website of the University of Illinois at Urbana-Champaign. This pattern quickly places the sensation within the reader that the child's innocence is not likely to remain, given the setting of the poem in Birmingham, a civil rights battleground. "But, mother, I won't be alone. / Other children will go with me, / and march the streets of Birmingham / to make our country free." The child's innocent questioning also moves the reader in the direction of cynicism, as the child's idea of safety in numbers will ultimately prove naive.
There Is No Sacred Place
Just as the mother in the ballad knows that the child couldn't possibly be safe in a crowded civil rights march, the reader is likely to have an unsettling feeling that all will not end well in church, either, which is indeed the case. The use of the church in the ballad emphasizes the point that in the battle of the times, no place is sacred, no place is safe. Since churches have historically offered sanctuary in times of war, the idea that an innocent child could be callously killed in this environment drives home how utterly vicious the opponents of civil rights could be.
Darkness and Tears
The poem uses images of darkness and light to contrast the civil rights struggle with innocence. The mother brushes her child's "nightdark" hair, and places white shoes on her feet. When the mother hears the explosion, her eyes grow "wet and wild," symbolizing tears of foreknowledge and the devastating angst of knowing that she lives in a world where her child's life has no value to the people in power in her community. Finally, the child's shoe is dug out of the rubble. The reader imagines that it is no longer white -- innocence has been destroyed.