In the first line of the poem ''To the doctor who treated the raped baby and who felt such despair,'' the speaker asserts that he/she is speaking...

Your question is a very good one.  Whenever you read poetry, it is important to note every single word and to understand the meaning and the context. 

There are clues throughout the poem to answer your question about the meaning of the phrase, "on behalf of us all."

First, the "us" appears to mean members of the family of the baby in question. 

In line 8 and line 11, there are references to the baby's mother.

Line 14 refers to "a bleary-eyed uncle."

Line 18 refers to the "grandpa who walked up and down with a colicky crier."

Line 21 reports that "a father sat watch."

If you try to imagine the scene, the injured baby has been taken to the doctor, and the poem's speaker is talking about the whole family who is concerned.  Each of these references lists some action that different family members usually take in the welfare of a baby -- changing it, walking with it, feeding it.  This gives the impression that this is a close-knit, caring family  that needs to work together.

In the closing four lines, the reference to "us" and "we" is repeated, but this time it seems different.  This time, the "rest of us" and "we slept in trust" seem to have a broader application, suggesting that the whole world sleeps better knowing that there are doctors like this one, ready to sacrifice themselves to help innocents like this baby.