Rationality is not equally distributed between adults and children / infants. They lack experience, education and the biologically-based capacity for rationality (ie no sense of object permanence in infants, mixed sense of conservation of matter in children). This disparity is part of how we adults can convince young people to do as we prefer (eat, wipe, not stab, etc.). So in this instance a disparity of rationality is not entirely or even mostly a bad thing.

But isn't the desirability of adults being able to convince children to do as they prefer a consequence of their lesser rationality? After all, if children knew as well as adults, there would be no reason not to let them make their own decisions.