It's hard to think of God sending babies to Hell...so I can't. But If I think that all babies would go to hell and God chooses to save some, that helps but still I don't completely understand how God works.
For instance, I don't understand this either, but it is what it is.
God in the Bible allows slavery, including selling your own daughter as a sex slave (Exodus 21:1-11, child abuse (Judges 11:29-40 and Isaiah 13:16, and bashing babies against rocks & Psalms 137:9)
Murder Men, Women, and Children
"These men are friendly with us; let them dwell in the land and trade in it, for behold, the land is large enough for them; let us take their daughters in marriage, and let us give them our daughters. Only on this condition will the men agree to dwell with us, to become one people: that every male among us be circumcised as they are circumcised. Will not their cattle, their property and all their beasts be ours? Only let us agree with them, and they will dwell with us." And all who went out of the gate of his city hearkened to Hamor and his son Shechem; and every male was circumcised, all who went out of the gate of his city.
On the third day, when they were sore, two of the sons of Jacob, Simeon and Levi, Dinah's brothers, took their swords and came upon the city unawares, and killed all the males. They slew Hamor and his son Shechem with the sword, and took Dinah out of Shechem's house, and went away. And the sons of Jacob came upon the slain, and plundered the city, because their sister had been defiled; they took their flocks and their herds, their asses, and whatever was in the city and in the field; all their wealth, all their little ones and their wives, all that was in the houses, they captured and made their prey. Then Jacob said to Simeon and Levi, "You have brought trouble on me by making me odious to the inhabitants of the land, the Canaanites and the Per'izzites; my numbers are few, and if they gather themselves against me and attack me, I shall be destroyed, both I and my household." But they said, "Should he treat our sister as a harlot?" (Genesis 34:21-31 NLT)
But I do know that at 3-5 years old, children begin to know right from wrong. My little boy tried to lie to me when he was 4. He sinned and knew he was lying. So are you saying that only until a baby begins to speak, thier salvation is secure?
I prefer to stick to not understanding God and His ways...but just having to trust Him.