bill and denise richard wrote an op-ed opposing the death penalty for tsarnaev in the boston globe on thursday .
the parents of 8-year-old martin richard , the youngest bombing victim , say they can only begin to move on once tsarnaev 's legal battle is over .
they want prosecutors to sign a plea deal , sending tsarnaev to prison for the rest of his life and forfeiting his right to appeal .
last week , tsarnaev was found guilty on all counts related to the april 15 , 2013 bombing at the boston marathon finish line .
next week starts the penalty phase of the trial , in which jurors will decide tsarnaev 's sentence .