The purity of a child is not praised because the children cannot behave in a bad way. In spite of the habit of viewing the child as an inspiration for gentle feelings, it learns fast all the gestures, activities and finally the words which are generally condemned in the case of an adult person.
To excuse them because it would not have a definite will, it is not a proper excuse. If the will of an adult is characterized by its firm and even violent expression, the children has this capacity, too. On the contrary, the aging makes us to temper the primary violent impulses.
Actually, the purity of a child rather refers to its lack of argumentative skills. And the parents accept the bad facets of their children exactly because their relation is not bolstered by arguments, but by nature.
Thus, the purity of a child means the purity of good and bad deeds, not only of the good ones.
If anybody was able in his childhood to be purely good and bad before acquiring argumentative skills, then the arguments and reasons the adults bring forward for their good and bad facts - when they are clearly good or bad - cannot be considered as their causes. They must be only subsequent excuses.