Whether the Web is making us smarter or dumber, isn’t there something just unconvincing about the idea that an occult “span” in the brain makes certain cultural objects more compelling than others? So a kid loves the drums but can hardly get through a chapter of “The Sun Also Rises”. The actions of children may dismay or please adults, but anyone who has ever been bored by one practice and absorbed by another can explain kids’ choices more persuasively than does the dominant model, which ignores the content of activities in favor of a wonky span thought vaguely to be in the brain.