Please explain the last line: "After the first death, there is no other." I thought it had to do with killing children during the war and once you start killing one child, it gets easier and easier to continue killing children.
The last line does not mean that it gets easier and easier to continue killing children. Instead, the speaker is saying that mourning the death of child kills the child a second time by normalizing the death with grief's platitudes. He is not going to brush away the reality of the "first death"--the real death--by "murdering" its "majesty" with mourning.
This is a poem that reflects with emotional anguish on the death of children during the German bombing of London during World War II. It does so by focusing on one child, a girl, universalized into "London's daughter." One child is mourned, but she represents all children killed.
Thomas doesn't want to trivialize or "murder" any child's death with false pieties, but he does want to remember the dead. He will not mourn: he will remember. In the last stanza he envisions her as part of her...
(The entire section contains 259 words.)