Bereavement is unavoidable but many religions promise an afterlife.
Child bone cancer and like are difficult to explain away. Perhaps we would have a cure for cancer if we didn't spend so much effort developing weapons and better advertising algorithms, but that is besides the point. I have heard Muslims believe that Allah will not inflict suffering on someone beyond what they can handle.
God, if they exists, could have created world we everyone were in a state of continually bliss, but what would be a point of that?