It is useless to obtrude moral ideas at an age at which they can evoke no response, and at which they are not yet required for the control of behaviour. The only effect is boredom, and imperviousness to those same ideas at the later age when they might have become potent. That is one reason, among others, why the study of child-psychology is of such vital importance to education.
Source: On Education, especially in early childhood, 1926, Pt. 2： Education of character, chap. 5: Play and fancy
More info.:http://russell-j.com/beginner/OE05-070.HTM