Unless the persons involved are in fact avoiding answering the question.
If only God knows whether Calvinists have reprobate children, then Calvinists can have no assurance that their children are elect. But if Calvinists can have assurance that their children are elect, then they must know that their children are not reprobate. Your reply does not designate which of those two propositions is true. And therefore my "Which is it?" question remains answered.
-A8
If, by some miracle, you met Paul today and he said to you, “I know whom I have believed,” I think you would question his assurance, too, and for the same reasons you question mine.
What you are discussing is a finer, and more specific, point of Calvinism; one held by the Presbyterians and not by the Reformed Baptists. I don't subscribe to infant baptism as I believe it to be a misguided doctrine of the Catholic Church.
If I understand the argument from the Presbyterians correctly, while they take a slightly different bent on the reason to baptize infants, it's not too much different than the Catholics. That is why I cannot follow your argument. If I understand the Catholic position, the reason they baptize infants is to bring them inside the Church while the Presbyterians bring them into the covenant. Infants who are baptized in the Church and die are assured of going to heaven, isn't that the teaching? Isn't the Presbyterian and the Church's views, while coming at it with different reasoning, one in the same? Catholics can't have any more assurance that the children they baptize are not reprobates any more then Presbyterians.
At the risk of treating this serious matter in a seemingly cavalier way, Reform Baptists believe immersion is a sign of our inward regeneration. It's an after-the-fact type ordinance rather than a forward looking ordinance. That doesn't mean that reprobates will not be baptized; it just means that it illustrates our rebrith and we're following in a command of Christ.
Our assurance of salvation rest upon faith in Christ; faith that God imparts to us. Children are saved in exactly the same manner and it depends upon the grace of God, not the will of man. Since faith is the substance of things not seen, we don't know who has faith and who doesn't, now do we? If we can't tell who's a false prophet, what makes us think we can tell a reprobate child from an elect child? Baptists generally will dedicate children but they will not baptize them until later, and then only after they undergo a series of questions and, sometimes classes. It is taken rather seriously.
While Calvin was a brilliant man of God, he was wrong on this point. Well, nobody's perfect. The Catholic Church is equally wrong but Calvin is a bit more excusable.