This seems like the most reasonable answer to the salvific will/unbaptized infant dilemma but it still raises the question of why God did not allow the infant to be baptised before it died (at least in the cases where the parents would have baptised the child had it lived).
Because salvation is a gift, no one deserves it. And humans have free will, these people murder their own unborn children, since that is their will God allows it and does mercy by allowing the unbaptised infant into limbo. Had the child lived and grown up they would have gone to hell. And if you ask why not baptised without growing up? We look at it in reverse, the fact that they are in limbo means they weren't among the elect so think of the implications.