During one of my interactions at a training center for priests and nuns about a decade back the medical director of a well known medical college said this:
If you want to pray go to the church, if you want treatment go to the hospital. Dont go to the hospital to pray and dont go to the church for treatment!
I did not read all the replies, so I might be saying the same thing as everyone else.
I think that the parents have a right to pray for their child's recovery, as useless as it may be. However, when it is clear that this is not a common cold, and the child requires medical services, the parents should be held accountable if those services are not used and the child loses life or limb. I think manslaughter is the appropriate charge, and I would add negligence and child endangerment to the charges if it results in longer sentencing. They should also have a stipulation that after time served, they will not be allowed to have any more children. Either mandatory sterilization or adoption. Any surviving children should be taken away as well and not given back. I know, rough.
Pray all you want but at the point of child endangerment intervention must happen and their actions (or lack of) are punishable by law, the problems arise in the process of enforcement. It's not about legalizing prayer for healing their children, but how to enforce punishment for their inaction (hopefully before death!)
I would phrase it slightly differently. I would say, it should be illegal for parents to deny actual medical care to their children, on the basis of religion or any means of "alternative medicine" like homeopathy, ayurveda, unani etc (or on any other basis for that matter, except if they cannot afford it).
In addition to real medical care, the parents can go to temples and break as many coconuts as they want. It doesn't hurt anyone, and helps the coconut merchants.