The Daily Mail (U.K.) reports:
Syed Zaidi faces up to ten years in prison for forcing [two] teenagers to whip themselves using a bundle of chains with sharp curved blades attached.
The device, called a zanjeer [for a photo, click on the link -EV], has been used for centuries by Shia Muslim men and boys to mourn the death of the Prophet Mohammed's grandson.
Zaidi lashed himself "until his bare back was covered in blood," then told a 15-year-old boy to do the same, and then "grabbed a 13-year-old boy by the arm, pulled off his T-shirt and made him flog himself as well. Both boys later needed hospital treatment for deep cuts to their backs."
It's apparently illegal in England for minors under the age of 16 to take part in such activities, even voluntarily (though of course it's often hard to tell with minors when such a decision is voluntary).
My sense is that the result is quite right on these facts, because this sort of behavior may well cause serious injuries. See also this post of mine about potentially, though rarely, life-threatening infant circumcision practices.
At the same time, the broader question of when it's improper to pressure children to engage in religious acts that are potentially physically painful or uncomfortable is a harder one. Fasting, for instance, is quite uncomfortable, which is one reason that people engage in it for religious reasons.
I'm inclined to say that it shouldn't be a crime to pressure your teenage children to fast, and that it certainly shouldn't be a crime simply to let them fast voluntarily -- again, noting the difficulty in determining voluntariness in many such situations -- despite the discomfort this might cause (unless the teenager has a medical condition that makes such fasting particularly dangerous). So my tentative sense is that the main distinction ought to be between (a) action that causes considerable physical pain or a significant risk of serious injury and (b) action that causes a modest amount of physical discomfort. At the same time, this strikes me as a pretty complicated question, even if the answer in some situations (e.g., whipping with blades until blood runs down your back) is pretty clear.
An extra twist: The law generally lets parents expose their minor children to considerable risk of pain and injury in typical kinds of sports (such as football), and even lets parents pressure their children to participate in such events. Should this be a benchmark for the kinds of risks that would be allowed for religious practices, so that the law would permit religious rituals that involve the same or lesser risk of pain and injury as playing football (or whatever is the riskiest sport that is generally allowed for minors)?