Actually, there IS evidence that infants go to heaven when they die. In 2 Samuel, King David had an infant son who died.
'And he said, While the child was yet alive, I fasted and wept: for I said, Who can tell whether GOD will be gracious to me, that the child may live? But now he is dead, wherefore should I fast? can I bring him back again? I shall go to him, but he shall not return to me.' (2 Samuel 12:22-23 Emphasis mine)
There is nothing in that section of Scripture, in any of that verse's context, to indicate that that David is talking about the afterlife. There is to indicate it's talking about one's fate. Sheol was the grave, and to them it was used as a place. Regardless of whether one was justified, they still went to Sheol.
Besides that, the Hebrew indicates that it is talking about the physical sense, not the spiritual sense:
H7725
שׁוּב
shûb
BDB Definition:
1) to return, turn back
1a) (Qal)
1a1) to turn back, return
1a1a) to turn back
1a1b) to return, come or go back
1a1c) to return unto, go back, come back
1a1d) of dying
1a1e) of human relations (figuratively)
1a1f) of spiritual relations (figuratively)
1a1f1) to turn back (from God), apostatise
1a1f2) to turn away (of God)
1a1f3) to turn back (to God), repent
1a1f4) turn back (from evil)
1a1g) of inanimate things
1a1h) in repetition
1b) (Polel)
1b1) to bring back
1b2) to restore, refresh, repair (figuratively)
1b3) to lead away (enticingly)
1b4) to show turning, apostatise
1c) (Pual) restored (participle)
1d) (Hiphil) to cause to return, bring back
1d1) to bring back, allow to return, put back, draw back, give back, restore, relinquish, give in payment
1d2) to bring back, refresh, restore
1d3) to bring back, report to, answer
1d4) to bring back, make requital, pay (as recompense)
1d5) to turn back or backward, repel, defeat, repulse, hinder, reject, refuse
1d6) to turn away (face), turn toward
1d7) to turn against
1d8) to bring back to mind
1d9) to show a turning away
1d10) to reverse, revoke
1e) (Hophal) to be returned, be restored, be brought back
1f) (Pulal) brought back
Part of Speech: verb
At best for you, it is inconclusive. At worst for you, it is referring to physical death, as context indicates. I'd stick to the former personally.
What did David mean that he would go to his infant son? Through inspiration, David documented that his own eternal destination was going to be 'in the house of the Lord'(Psalm 23:6). It is clear that David believed his son went to heaven. And he would one day be reunited with him.
That is an assumption based off of nonexistent implication. See above.
The Bible says that the soul who sins will die. Not the soul who has a sin nature. It is actual sins commited that damns a soul. And since babies don't understand the law, they can't break what they don't understand.
Sin is sin, regardless of understanding. I've already demonstrated this. Here's the Greek for the word sin in 6:23:
G266
ἁμαρτία
hamartia
Thayer Definition:
1) equivalent to 264
1a) to be without a share in
1b) to miss the mark
1c) to err, be mistaken
1d) to miss or wander from the path of uprightness and honour,to do or go wrong
1e) to wander from the law of God, violate Gods law, sin
2) that which is done wrong, sin, an offence, a violation of the divine law in thought or in act
3) collectively, the complex or aggregate of sins committed either by a single person or by many
Part of Speech: noun feminine
The same is used in 7. Now, given the three different meanings behind the word, 2 would be what Paul would be talking about in context, not 1 or 3 as 6:23 and 3:23 use in context.
You mentioned that you believe that babies don't go to hell. You are right. That would make God a cruel monster. But they have souls. So if they don't go to heaven, where else would they go? They have to exist somewhere. Are they just floating in outer space? Do they turn into angels? Do they become extras in the cartoon 'Rugrats'? Just pondering.
Read this very clearly, I will not say it again. We don't know. Anything we might say is speculation.