The problem with quoting the Old Testament regarding unbaptized babies going to heaven is though Christ existed before the "Word was made flesh" the sacrament of baptism had not yet been instituted so we are not dealing with unbaptized children in the OT. We are really comparing apples to oranges.
This is just my thought but I look at it this way: faith comes through hearing, so a dead child (a stillborn or a child that has died before the age of cognicent thought)would not have known of Jesus Christ and had no input on whether to deny Christ and reject baptism. So, in this case the child would not be guilty of disbelief (unbelief or whatever you want to call it) and this unbelief is what really damns the soul - this rejection of Christ.