Most monotheists beleive that it is only God who can endow the body with the soul. So if God is opposed to non-monotheistic religions, why would he send souls into babies that he knows are going to be raised as pagans?
Now lets apply your viewpoint logically.
If God allows a baby to be born into a family that has a religion that practices the sacrifice of babies, then God must approve of a religion that practices the sacrifice of babies, since he allowed a baby to be born into that particular family that practices that religion.
Just as I said before, you are saying that if God allows it (in this case He allowed the family involved in a religion of baby sacrifice to produce offspring), then God must not have a problem with a religion that practices baby sacrifice.
I of course disagree with your viewpoint. I don't think a religion is justified by the fact that members can have children.
Things you are missing here:
1) Nobody ever said those born into pagan households would all remain pagan forever. There is a thing called "religious conversion."
2) You seem to be of the impression that pagans serve no purpose in God's plan.
3) God never guarantees that all souls are created to be "saved."
4) We have free will. You seem to be arguing that it can not be possible for God to desire all men to be Christians because He very clearly created a world in which all men will not be Christians. This is to totally miss free will.
We are not actors on a stage playing out lines God pre-scripted.