Where does prevenient grace originate in the Bible? What is the Biblical basis for Prevenient Grace?

I am specifically referring to the belief that if a child is born and dies before reaching the age of responsibility, and so did not have the opportunity to accept Christ, that child will still go to heaven.
From a Lutheran perspective:

We do not believe in us having to do anything.  Even accepting Christ.

Ephesians 1:4


  4 For he chose us in him before the creation of the world to be holy and blameless in his sight.


He chose us.  Not the other way around.

If we don't accept Christ, how do we know that we're saved?

Baptism.

1 Peter 3:21


  21 Baptism now saves you


Only God can really know whether someone goes to heaven or hell, but if the child were baptised, I would say that 1 Peter 3:21 provides a good basis for the belief that they could still go to heaven.