I think as a parent you would know when a child is ready to take this step. A story once told to me about a five year old boy who was kidnapped by thieves and was trained to steal and he kept telling them he would not do it. He did as they instructed in the training but at the same time he kept telling them he would not help them steal and witnessed to them about the kingdom and his knowledge of God. When they took him to the store he did everything they said until he had to actually steal and he refused so they left and once again they beat him but this time they beat him so bad they thought they had killed him. They then took his body to the garbage dump and threw him away like so much garbage but he was still alive and when the truck came to dump the driver seen something move and realized it was a little boy. He survived. This little five year old boy was informed enough and love his God enough to be baptized. This maturity come at different ages and the parent would be the one that knows when that time comes in my opinion.
Happiness keeps you sweet, Trials keeps you strong, Sorrows keeps you human, Life keeps you humble, Success keeps you glowing, But true friends keep you going.