Children who are not suppressed ask innumerable questions, some intelligent, others quite the reverse. These questions are often wearisome, and sometimes inconvenient. But they must be answered truthfully, to the best of your ability. … Tell him rather more than he can understand, not rather less ; the part he fails to understand will stimulate his curiosity and his intellectual ambition.
Source: On Education, especially in early childhood, 1926, Pt. 2： Education of character, chap. 8: Truthfulness
More info.:http://russell-j.com/beginner/OE08-070.HTM