The dogs, clubs, hoses, guns, and jails of stanza 2 all serve as symbols of the police, who regularly broke up the Freedom Marches. Instead of mentioning the officers themselves, Randall associates them with the tools they used to stop the marches. That's a case of what's called metonymy. It is as if the officers aren't individuals, but part of a force of repression and violence in the poem.
To the mother, the police are a danger to her child, and these tools symbolize that threat. Later, in stanzas 7 and 8, the rubble of the church symbolizes the end result of the violence. For a poem ultimately about the destruction that bigotry can cause, these violent images serve to remind the readers that even children weren't safe from the potentially deadly effects of racism.