The author concludes that it is necessary for American children to do calisthenics on the grounds that European kids do them, and are more fit than American kids. However, this is flawed as we do not know if calisthenics are the cause of better fitness in europe- nor do we know if there are other ways (maybe aeropics, or better exercise) that we could improve the fitness of American kids.
This is a necessary assumption question, and there seem to be two big necessary assumptions- one being that the calisthenics are a critical part of why european kids are fit, and the other being that there is no other way for american kids to get fit.