I'm not so sure that the parents of that child would think God failed them at all. Of course I haven't talked to them, but I have heard others state that they have totally given their child to God. Whether the child lives or dies, God is still God. That is the case whether they took the blood or not. It is sad when a parent looses a child. I could not imagine what that would be like. However, I bet they lean heavily on Job 2:10 in a situation like this.
They don't reject all medical intervention. They just draw the line with things like accepting blood from another person. As someone else already mentioned, others don't vaccinate their children. Some shun medical attention in situations that I never would, like having a baby at home.
They see it as the way things should be. I feel like even if I decided to do it with no drugs or other interventions, I want to be in the hospital just in case something doesn't go according to plans. Should those people be executed should their child die?