In the Bible Luke ministered in medicine right alongside Paul's miracle ministry, this is documented on Malta, but we can assume it was probably the case wherever they travelled together. Jesus never told anyone not to seek a doctor, except the Pharisees who were already healed. Often when Jesus, or the Apostles healed someone the Scripture says seeing that the person had the faith to be healed (or some such thing) they laid hands on them. Lack of faith may be a sin, but dying in your sins is much worse than seeking medical help, In fact when a good doctor uses their knowledge to do good arre they not ministering God's love. Now we just need to get a healthcare system that allows everyone ot have access to the care they need. A few honest to goodness miracles would not be bad either.
What punishment should these parents get? Since there is good reason to suspect that the church has seen many such deaths over the years, you could make the case that a rational person would suspect it might happen again and that since the type of faith the church employs is clearly not 100% successful allowing for medical treatment of infants and children might actually be a reasonable act of concern. Therefore, the parents are either suffering from a form of mental illness (either already present and/or induced by the cult-like atmosphere of the church) which would insist on some type of criminal insanity case, or they are competent to stand trial for negligent homocide.
Either way they should not be allowed to be in the care of children probably for the rest of their lives at least, even if paroled and action should be taken by the State to assess the overall suitability of all members of the church relative to the care of children and infants.
I'm guessing the defense will argue that it is the belief of the defendants that toddlers go to heaven, not having reached an age of accountibility. That would be a key in this, because if the parents hadn any inkling that their children might suffer and undesirable afterlife if they died not having purposefully accepted the faith, you could argue insanity or cruelty (which I think is already a given, they refused to take steps to ease the suffering of a dependent in their care when those steps were both known and readily accessible.) Since as I said, a rational person would know that prayer does not always heal, and a sane loving parent would want their child to live out his/her life.
How long? Life if that is what it takes to make sure that someone who is going to sit and watch a baby die when help was five minutes away never gets to have another baby.
But this case has so many possible aspects. It has civil liberties in several angles: religious freedom, mental competancy and the rights of the mentally ill, parental rights, big brother type aspects, social responsibility as it pertains to legal jeopardy. And then if you focus on the rights of the child, you could see a verdict that has some bearing on abortion rights as well.
However things turn out, there is no excuse for letting your baby die when you could have done something about it, and if these parents ever actually come to Christ the guilt they have to deal with will be severe and lifelong I am sure. God can forgive anyone, the enemy never forgets and we rarely when honest make it easy on ourselves.
Lord I pray that you would bring to an end the harm this church is inflicting upon it's people in Jesus Name Amen.