Vayikra 

ויקרא

There is an ancient custom for children to begin their Torah study with the book of Leviticus, which addresses the laws of the offerings, consecrations, and ritual purity and impurity. The Sages explain that there is no one more appropriate to engage in these matters than children. 

Rabbi Asya said: Why do young students begin their studies with the book of Leviticus? It is because all the offerings are written there, and because the children are still pure, as they have not yet tasted sin. Therefore, the Holy One, blessed be He, said that children should begin their studies with the order of the offerings – let the pure come and engage in the act of the pure [i.e., offerings, brought by those who are ritually pure].  Therefore, I ascribe credit to them as though they are standing and sacrificing offerings before Me. And the verse informs you that now that the Holy Temple was destroyed and the rite of the offerings is not performed, leaving the world without their merit, were it not for the young children who read the order of the offerings as a substitute for bringing them, the world would not endure. <source>(<i>Tanĥuma</i>, <i>Tzav</i>) </source>