The poem begins with an inscription that alerts readers to important contextual information—namely, that "Ballad of Birmingham" is based on the bombing of a church in Birmingham, Alabama, in 1963. This event was an important and devastating moment in the Civil Rights Movement, when white supremacists bombed a predominantly Black church called the 16th Street Baptist Church, ultimately killing four young Black girls in the process.
With this in mind, the first stanza makes it clear that the poem features a conversation between one of these young girls and her mother, as the girl asks if she can go "downtown" to join a demonstration for racial equality (referred to here as a Freedom March) instead of going out to "play." Right away, a notable juxtaposition arises between the childish act of playing and the more mature act of protesting. Indeed, the daughter seems eager to leave behind childish games so that she can march in the streets and fight for racial equality like an adult.
The early 1960s were tumultuous but incredibly important in southern cities like Birmingham, as Civil Rights leaders like Dr. Martin Luther King Jr. called for nonviolent action to stand up to the racism that was so pervasive at that time (and is unfortunately very much alive in contemporary society, too). Part of Dr. King's efforts in Birmingham included recruiting politically active young people from nearby schools, calling upon Black children to join marches to protest the city's segregationist policies and racist police commissioner.
This, then, is most likely why the daughter in "Ballad of Birmingham" is so eager to become involved in the activism taking place in her city. In order to convince her mother, she calls her "Mother dear," as if doing this will endear her to her mother. Accordingly, readers will perhaps sense that the daughter expects her mother to say she can't go to the marches, so she addresses her affectionately in the hopes of convincing her. It's also the case that calling her mother "dear" makes the daughter sound somewhat sophisticated, once more suggesting that she wants to distance herself from childish behaviors in order to more fully enter the adult world. In this sense, it's possible that her mature tone is an attempt to convince her mother that she's mature enough to participate in an important protest.
In keeping with this, the young girl uses certain poetic devices to further enhance the sound of her speech, making her sound even more sophisticated and mature. For instance, she uses alliteration throughout the first stanza, repeating the /m/ sound in words like "Mother," "may," and "march." In fact, she also uses consonance to double down on this /m/ sound, meaning that the sound is quite prominent:
"Mother dear, may I go downtown
Instead of out to play,
And march the streets of Birmingham
In a Freedom March today?"
In addition to this alliteration and consonance, the girl also uses assonance to repeat the long /ee/ sound in words like "dear," "streets," and "Freedom." This, in turn, adds a certain musicality that makes her sound even more adult, which is the exact effect she hopes her mother will register, since this might convince her mother to let her go to the march.
On a technical level, it's also worth noting that these first four lines establish the poem's overall rhythm, since they are written in common meter, a meter in which the lines alternate between iambic tetrameter and iambic trimeter. A line of iambic tetrameter includes four iambs, which are metrical feet consisting of an unstressed syllable followed by a stressed syllable (four da-DUMs). A line of iambic trimeter, on the other hand, includes only three iambs (three da-DUMs). Lastly, these lines also establish the poem's ABCB rhyme scheme, which is a common scheme used in ballads like this one.