It's a jungle in here. Or something like that. In the poem, animals are associated with killing and weapons. The speaker uses animals in order to suggest that there is something inhuman about warfare and the murder that goes with.
Line 2: The bayonet is "keen with hunger of blood" in a metaphor that compares the bayonet to a bloodthirsty predator.
Line 10: There are "no claws" behind the boy's "fingers supple." Well that's good, we guess. Note the contrast here, between the boy's soft and gentle hands, and the hard, animal nature of claws.
Line 11: The speaker says "God will grow no talons" on the boy's heels. Again, our attention is being drawn to the difference between this young kid, and predatory animals.
Line 12: God will not grow "antlers" on the boy's head either. This, too, is a metaphor for the fact that the boy will never become a brutish animal. Phew, that was a close call.