I can see both sides of this debate. This judgement involves a lot of subjective and arbitrary standards, including the degree of the superiority of chemotherapy, the competence of the child to make his own medical decisions, and even whether the benefits of the treatments are worth the suffering. (If diagnosed with a particularly aggressive form of cancer that would require frequent chemotherapy treatments, I might want to skip the prolonged-but-painful life and just make the most of the time I had left while I still felt pretty good.)
The government has no business forcing treatment on an independent citizen, except in cases of dangerously contagious illnesses and quarantines. These parents might be violating the child's rights, however, since they have the power to grant or deny treatment, and profoundly influence the child's opinions as they please. Essentially this is a question of the level of ownership that the parents can claim.