Children are not supposed to fast, but we help them learn to fast and train them in it a couple of years before they reach the age of bar/bat mitzvah. Is that training a requirement? Does it mean the kids must fast in preparation, or are they easing into fasting by eating later than usual. Also, a pregnant woman who has a strong craving - you can feed her the food she wants, even on Yom Kippur. Certainly that's true of someone who is ill and thinks food is the cure too. The issue, of course, is the primacy of life and pikuach nefesh as a backdrop to even a pregnant woman's cravings. And then the Gemara talks about the exceptions to the principle of pikuach nefesh taking precedence - namely, adultery, idolatry, and murder. Here, the rational for giving up ones life rather than violate adultery (really, all sexual illicit violations) and murder, which have different reasons...