These lines exhibit at least two different figures of speech.
First, this description demonstrates alliteration with a heavy emphasis on the repetition of the s sound: "stout of / Step." This repetition has a shushing effect, demanding that readers demonstrate the respect necessary to honor these women who have fearlessly worked on behalf of their children. After all, these mothers have faced "headragged generals" and "mined / Fields" in order to secure "books" and "desks" for their children. The s sound is also often associated with hissing or danger, and here, the alliteration of this sound conveys the fierce spirit of these mothers on behalf of their children.
The lines are also an example of enjambment, which occurs when a sentence is continued on another line in verse (without a pause, such as is created by a comma or semicolon). This creates emphasis on the word step, a monosyllabic word that is isolated on a line through enjambment. This is a word of action, both simple in structure and powerful in connotation, and placing emphasis on it serves to illustrate that these mothers are actively moving forward, taking necessary steps to ensure their children will receive the education that they themselves were never afforded.