This is actually a red herring. The Scriptures are clear that all have a sin nature passed on by Adam. It is part of the curse.
As for the destiny of children, Abraham said: "Shall not the Judge of all the earth do right?"
Do you believe that?
Do you believe that God in his mercy will do right?
We throw ourselves on the mercy of God and believe that he will do that which is right concerning infants. Personally I believe that infants will be in heaven. I believe in a merciful God.
But I also believe that the sin nature is passed on from individual to individual through Adam. That is why it is called the "Adamic nature." That is clearly taught in Scripture.
The curse was on the earth. Show me where God cursed man's moral nature.
Gen 3:17 And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it:
cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life;
18 Thorns also and thistles shall it bring forth to thee; and thou shalt eat the herb of the field;
19 In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken:
for dust thou art, and unto dust shalt thou return.
God cursed the ground, not Adam. God said the earth would bring forth thorns and thistles. I would say this means hardship. Men must work hard to earn a living, food did not grow easily anymore.
God did say Adam would return to the dust, which is physical death, but there is not one word that says man's moral nature was cursed or affected.
Now, that said, God said man now knew good from evil. This is what makes man responsible, God does not hold little children who do not know good from evil responsible.
Deu 1:39 Moreover your little ones, which ye said should be a prey, and your children,
which in that day had no knowledge between good and evil, they shall go in thither, and unto them will I give it, and they shall possess it.
God did not punish the Jewish children when their parents sinned, because they did not know between good and evil in that day.
Jon 4:11 And should not I spare Nineveh, that great city,
wherein are more than sixscore thousand persons that cannot discern between their right hand and their left hand; and also much cattle?
God implied to Jonah that he SHOULD spare Nineveh, because there were sixscore thousand little children who could not discern between their right hand and left hand there.
So, it is absolutely clear that God does not hold children responsible for sin until they are mature enough to understand good from evil.
Isa 7:16 For before the child shall know to refuse the evil, and choose the good, the land that thou abhorrest shall be forsaken of both her kings.
This verse not only shows little children do not know between good and evil, it also shows that we do not HAVE to sin, and that even a child can refuse evil and choose good.
Jesus NEVER spoke evil of children, but said we must be converted and become as little children to enter the kingdom of heaven.
Mat 18:3 And said, Verily I say unto you,
Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
Anybody who says the scriptures teach little children are sinners does NOT know the scriptures. The scriptures teach the exact opposite.