The first thing to know about the child is if that child is able to understand the full extent of 1) The sacrifice of Jesus, 2) The love of God and 3) the full extent of what happens for rejecting Jesus.
While a young child cannot understand the full consequences of this rejection the child will not be held to a decision for or against. In other words, this child is innocent in God's eyes.
However, a teenager can understand full well the consequences of that rejection and will be held fully responsible for those actions.
At what age does this take place? It is different for each child. God knows the heart of the child, and the mind as well.
To throw another thought into this mix, what if a person is 40 years old but has a limited IQ and has the mind of a 4 year old? Once again, God know our hearts. He knows that a child of 4, no matter what physical age, cannot comprehend the full consequences of the decision for or against Christ.
Thanks for the clear explanation.