It's a good question, and I don't pretend to have an answer. But I will say this:
A 40-day fast? A child who starts to fade and seems to be starving to death? Well, I'd say that intentionally withholding food to the point of causing death is not acceptable. Our bodies require food at all points of our lives, and to withhold food from someone to the point of feath is immoral.
Human sacrifice? Physical harm? Again, I would say that these are wrong. To physically assault another human as a "method of worship" is not something I would want to justify.
Now, here's where I would draw the line: A modern medical technique, which can only be developed by an advanced, industrialized society ought to be optional.
Progress is fine for those who want it, but I don't think "progress" ought to be mandatory. I don't like the idea of government forcing whatever is latest and greatest on my family. I would never take the step of trying to withhold progress from your family, should they wish to take advantage of insulin or chemotherapy -- but if some family were to tell me that their relationship with God requires them to pass up the latest medical techniques, I just don't see how (or why) society should force them.
I would also say that this gets into socialized medicine and the cost of that. Cancer treatments are not cheap. I say I don't want it -- but government says I must have it. Okay. That means government will pay for it, right? Don't we run the risk of going down the road of unfunded mandates -- you MUST buy this level of healthcare! Or, alternatively, everyone's tax dollars will be used so that your sick child will get whatever medical care the government thinks is appropriate. Obama thinks like that -- but I don't.
On the basis of religious freedom, medical freedom, economic freedom, and personal responsibility, I think that choosing to pass up on medical care ought to be a legal option.
Okay, but passing up medical care on behalf on another who has no say so? Just as a person who claims to be a conscientious objector on religious grounds must defend that position with specific religious doctrine, the child’s parents must be able to defend THEIR belief that prayer and faith alone WILL work to heal their child. The Christian faith DOES NOT support that argument.