Children need to be taught that the Bible is where they can go to find out how to organize their life. Even if the children cannot read, I think it would be helpful just to expose them to the fact that the Bible does exist, it is truth, and they can trust Scripture.
Right doctrine is also important because these first lessons and interaction will become the foundation upon which children will come to understand faith, God, and salvation. If they do not have right beliefs, this will bring confusion, and leave them open to falling away from the church. Everything must be consistent.
Finally, evangelism is important, even at this early age. I have found that children are very influential and they teach each other. Some recent examples in my own life is with my daughter in her kindergarten class. My daughter did not know how to tie her shoes. I had tried to teach her, but we both would end up frustrated. When she went to school, one of her classmates taught her how to tie her shoes. In the same way, when we teach children that they should tell their friends about the Gospel, they can influence each other in a positive way.