This is a violent poem, no doubt about it. The woman whips the boy so hard that the stick she uses breaks. In fact, she's so set on whipping this poor little kid, she chases him all over the place and ignores his pleadings. On top of this, there's a violent memory nestled in the middle of the poem that involves somebody's head (the speaker's or the woman's) stuck between somebody's knees while it gets beaten. Violence in the present, violence in the past makes for some sad times, gang. Saddest of all is that there's a chance there will be violence in these people's future, too.
Lines 1-2: Fight away we learn that violence is a fact of life for the boy and the woman. She is whipping him again. Yep, she's definitely a child abuser.
Lines 5-6: The boy's crashing through the bushes here is definitely a violent kind of crashing—loud, confusing, and scary.
Lines 9-10: We get lots of S sound alliteration here to emphasize the woman's violent actions ("strikes and strikes the shrilly circling"). The stick also breaks, which underscores how extreme the woman's violence is.
Lines 13-15: We don't know for sure whose memory is described in these lines, but it's really violent, that's for sure. "Bony vise" here is a metaphor for the strength and destructiveness of the person's knees,
Line 22: The woman's violence against boy is her attempt to "purge," or eliminate her own painful memories. Her life is controlled by the past.
Lines 23-24: The woman's violence is also an attempt to get "revenge" for all the "hidings" she has had to bear. We're gonna go ahead and say that "hidings" here is a euphemism for something like "horrible, violent, painful memories that are so had they shouldn't even be spoken."