However, to be responsible for sin one has to understand it
You have Biblical confirmation of that?
John 9:41 Jesus said to the Pharisees, "If you were blind, you would not be guilty of sin; but now that you claim you can see, your guilt remains."
It would seem to allow all people into heaven and we really don't need Jesus then.
Incorrect.
As long as I don't understand that Jesus is needed to get to heaven then I'm ok.
In the end you may go to heaven, you may not... I cannot say, but the end is now. And right now you definately need Jesus to have life and live it to the fullest. If you never had the opportunity to hear of Jesus, God may have grace on you in the afterlife, but this life is just as important as the next.
A young child is not responsible for his sin, but rather his parents are responsible.
Again....in scripture somewhere?
Mat 18:6 "If anyone causes one of these little ones who believe in me to sin, it would be better for him if a large millstone were hung around his neck and he were drowned at the bottom of the sea.
Mat 18:7 How terrible it will be for the world because it causes people to sin! Temptations to sin are bound to happen, but how terrible it will be for that person who causes someone to sin!
How does this stack up with "For all have sinned and fall short of the Glory of God" and Jesus' words "No one comes to the Father except by me".
How does it not? Jesus died once for the sins of ALL. Jesus has the power to forgive sins. If anyone comes to the father it is because Jesus forgives them and Jesus has unlimited power to forgive.
There is sufficient reason from scripture (David's kid that died, and Jesus saying, "The kingdom of heaven belongs to such as these," etc.) to believe that God has mercy on children who have not had the opportunity to believe.
We are all sinful. None of us can deserve life. But God wants to give us all life. The way I understand it, the only people that will assuredly not recieve it are those who always reject the Giver of Life.