In How the Other Half Lives, Jacob Riis wrote: "With half a chance it might have lived; but it had none. That dark bedroom killed it." What does this mean?

It means that there had never been a chance at all for the child to survive.

Explanation:

Jacob Riis' "How The Other Half Lives" talks of the poverty and the dilapidated slum areas where lots of people stay. The passage in particular talks of a child suffering from measles and if given a chance, even a little bit of chance, with a better place or condition, he would have suffered. But the place had none so he was stripped of any chance to survive. This is representative of the whole of the new York slum areas where immigrants come in the hope of leading a better life but end up suffering more than they had in their own countries.