We frequently set limits not on religious beliefs but on their practices. The two determinant used there are the possible harm to others and whether the person being harmed can be deemed ‘capable’ in a legal sense. There can be no doubt that the decision to refuse available medical treatment causes harm, that is beyond dispute. The issue then is whether the person harmed, the child, can be considered capable. Legally they cannot, they cannot enter into a contract, they cannot marry or vote, legally they are not allowed to make many decisions because they are not full members of society until they are adults. It is worth noting that if the child is not deemed competent to make a decision regarding their own healthcare, it is difficult to see how their determination of their own religious choices can be assumed as authoritative. So the child cannot make the decision and the parents actions would cause harm to the child.  In the light of this, the only remaining opinion is that of the doctor.