If you think that babies of believers who die in infancy might go to hell, or might not, depending on the inscrutable sovereignty of God, then I have news for you. You’re afoul of the Canons of Dort, which gave us the Five Points of Calvinism.
That synod says, in article 1.17,
“Since we must make judgments about God’s will from his Word, which testifies that the children of believers are holy, not by nature but by virtue of the gracious covenant in which they together with their parents are included, godly parents ought not to doubt the election and salvation of their children whom God calls out of this life in infancy.”
Let me parse that further. Dort asserts that God has spoken about our children. Failure to believe what God has spoken is unbelief. Therefore, if you have lost a child in infancy or in the womb, it is an act of unfaith and sin for you to doubt whether God has saved that child.
The Synod of Dort was also concerned that certain “calumniators” were
“wishing to persuade the public that the doctrine of the Reformed churches concerning predestination…teaches that many children of the faithful are torn guiltless from their mothers’ breasts and tyrannically plunged into hell; so that neither baptism nor the prayers of the Church at their baptism, can at all profit them.” (Synod of Dort, Conclusion)
Let me parse that too: There are people who claim that the Calvinistic doctrine renders baptism an otiose ritual that doesn’t benefit babies who die. (Pretty much all baptists say this.) The Synod of Dort says that if you say that this is Reformed doctrine, you are a “calumniator”, and you should “consider the terrible judgment of God which awaits” you “for bearing false witness against the confessions of so many churches.”