Does God Save Children Apart From their Faith In Jesus?

I do not understand why anyone would think that age or mental capability is relative to salvation. There is no where in the word of God that shows man upon his own free will making the righteous choice over a period of time. All die in Adam. Death is the wages of sin.

(For until the law sin was in the world: but sin is not imputed when there is no law. Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come.

We ask questions about babies that die before a certain age whatever that age might be, because they aren't old enough to have understood. Think about Jesus born of Mary and for thirty three years was without sin, yet he died that very death spoken of above. He who knew no sin was made sin for us and died.

Just what death (there is only one kind) did he die? Here is the death he died and it is the same death all die.

Because thou wilt not leave my soul in hell (Hades), neither wilt thou suffer thine Holy One to see corruption.

Did David pray this prayer for himself or for the fruit of his loins the Christ?

What about the once living soul David. Fifty days after the resurrection of the fruit of his loins the Christ, where was the soul of David and his status concerning corruption?

Per Peter. Men brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day. For David is not ascended into the heavens:
Per Paul. Wherefore he saith also in another [psalm], Thou shalt not suffer thine Holy One to see corruption. For David, after he had served his own generation by the will of God, fell on sleep, and was laid unto his fathers (Sheol, Hades), and saw corruption:

The implication is the soul David is in Hades and he saw corruption.

Praise God, that by his grace he beget in the virgin Mary his only begotten Son Jesus and that his Son Jesus the man humbled himself, and became obedient unto death, even the death of the cross. Obedience of Faith.

By Grace are ye saved through The Faith.

The just shall live by His Faith. From Hab. 2:4 and understood by Paul as being The Faith of Jesus the Christ.

It is that faith which will save newborn's that die and old ones that die.