While "The Chimney Sweeper" by William Blake might have a singsong rhythm, rhyme, and feel to it, the poem is quite sobering and anything but happy. The poem has a light and airy feel, but that feel is definitely contrasted by the poem's topic. The poem is about extremely young boys being forced into horrific working conditions that can and do eventually kill them, and William Blake does not delay in letting readers know that this particular poem's topic is not light and happy.
Already in the first stanza Blake is letting his readers know that chimney sweeps are extremely young boys. The line in question tells readers that the boy was sold into the sweeping industry before his "tongue / Could scarcely cry." To be clear, tongues can't cry; therefore, Blake is substituting "tongue" for something else. This is metonymy, and Blake is using it in place of "voice" or "mouth." If the poem's speaker can't correctly utter the word "weep" or "sweep," then the speaker is extremely young. By telling readers that the narrator could scarcely cry those words, Blake is steering readers toward understanding that the boy's father sold his child into the harsh working world at a very young age.