I am still learning so any feedback would be useful here, but I do think I have some things to add to this discussion.
We need to keep in mind that in a technical sense, we are driven to the honest answer that if God chose to Save anyone they will be Saved. Since there is not a direct declaration about unborn children, or those with limited mental faculty, we are left to reason with what we know of His character.
Although David committed grievous Sin during his lifetime, God described him as a man after His own heart.
1 Samuel 13-
13 Samuel said to Saul, “You have acted foolishly; you have not kept
the commandment of the Lord your God, which He commanded you, for now
the Lord would have established your kingdom [f]over Israel forever.
14 But now your kingdom shall not endure. The Lord has sought out for
Himself a man after His own heart (speaking of David), and the Lord
has appointed him as ruler over His people, because you have not kept
what the Lord commanded you.”
David's life is interesting indeed, but in reference to unborn children - David lost a child, as explained:
2 Samuel 12-
7 Nathan then said to David, “You are the man! Thus says the Lord God
of Israel, ‘It is I who anointed you king over Israel and it is I who
delivered you from the hand of Saul. 8 I also gave you your master’s
house and your master’s wives into your [e]care, and I gave you the
house of Israel and Judah; and if that had been too little, I would
have added to you many more things like these! 9 Why have you despised
the word of the Lord by doing evil in His sight? You have struck down
Uriah the Hittite with the sword, have taken his wife to be your wife,
and have killed him with the sword of the sons of Ammon. 10 Now
therefore, the sword shall never depart from your house, because you
have despised Me and have taken the wife of Uriah the Hittite to be
your wife.’ 11 Thus says the Lord, ‘Behold, I will raise up evil
against you from your own household; I will even take your wives
before your eyes and give them to your companion, and he will lie with
your wives in [f]broad daylight. 12 Indeed you did it secretly, but I
will do this thing before all Israel, and [g]under the sun.’” 13 Then
David said to Nathan, “I have sinned against the Lord.” And Nathan
said to David, “The Lord also has [h]taken away your sin; you shall
not die. 14 However, because by this deed you have given occasion to
the enemies of the Lord to blaspheme, the child also that is born to
you shall surely die.” 15 So Nathan went to his house.
Loss of a Child
Then the Lord struck the child that Uriah’s [i]widow bore to David, so
that he was very sick. 16 David therefore inquired of God for the
child; and David fasted and went and lay all night on the ground. 17
The elders of his household stood beside him in order to raise him up
from the ground, but he was unwilling and would not eat food with
them. 18 Then it happened on the seventh day that the child died. And
the servants of David were afraid to tell him that the child was dead,
for they said, “Behold, while the child was still alive, we spoke to
him and he did not listen to our voice. How then can we tell him that
the child is dead, since he might do himself harm!” 19 But when David
saw that his servants were whispering together, David perceived that
the child was dead; so David said to his servants, “Is the child
dead?” And they said, “He is dead.” 20 So David arose from the ground,
washed, anointed himself, and changed his clothes; and he came into
the house of the Lord and worshiped. Then he came to his own house,
and when he requested, they set food before him and he ate.
21 Then his servants said to him, “What is this thing that you have
done? [j]While the child was alive, you fasted and wept; but when the
child died, you arose and ate food.” 22 He said, “While the child was
still alive, I fasted and wept; for I said, ‘Who knows, the Lord may
be gracious to me, that the child may live.’ 23 But now he has died;
why should I fast? Can I bring him back again? I will go to him, but
he will not return to me.”
Earlier in the text, it is clear that David is Saved because he is told by Nathan that despite his horrible Sin, it had been washed away by the Lord. So, we know multiple things here.
1) David was Saved despite his Sin, because of the Lord's washing it away.
2) God is the one who killed the child. (God kills 100% of everyone, of course).
3) David, in his response to God's Justified Judgement, makes it clear that he fully expects to one day be with his now dead child.
It cannot be directly stated as I said before that this is the case with all children, surely it is up to the Lord; however, in principle we see here that a Saved man is completely assured he will be reunited with his dead child. He is not corrected or chastised for this either...
So, the only other things we can go on are the understanding of God's character and how He 'may' apply Election to those in unfortunate circumstances. Again, I cannot speak precisely, only in principle; for lack of direct Scripture on the subject.
Genesis 18-
22 Then the men turned away from there and went toward Sodom, while
Abraham was still standing before the Lord. 23 Abraham came near and
said, “Will You indeed sweep away the righteous with the wicked? 24
Suppose there are fifty righteous within the city; will You indeed
sweep it away and not [t]spare the place for the sake of the fifty
righteous who are in it? 25 Far be it from You to do [u]such a thing,
to slay the righteous with the wicked, so that the righteous and the
wicked are treated alike. Far be it from You! Shall not the Judge of
all the earth [v]deal justly?” 26 So the Lord said, “If I find in
Sodom fifty righteous within the city, then I will [w]spare the whole
place on their account.” 27 And Abraham replied, “Now behold, I have
[x]ventured to speak to the Lord, although I am but dust and ashes. 28
Suppose the fifty righteous are lacking five, will You destroy the
whole city because of five?” And He said, “I will not destroy it if I
find forty-five there.” 29 He spoke to Him yet again and said,
“Suppose forty are found there?” And He said, “I will not do it on
account of the forty.” 30 Then he said, “Oh may the Lord not be angry,
and I shall speak; suppose thirty are found there?” And He said, “I
will not do it if I find thirty there.” 31 And he said, “Now behold, I
have [y]ventured to speak to the Lord; suppose twenty are found
there?” And He said, “I will not destroy it on account of the twenty.”
32 Then he said, “Oh may the Lord not be angry, and I shall speak only
this once; suppose ten are found there?” And He said, “I will not
destroy it on account of the ten.” 33 As soon as He had finished
speaking to Abraham the Lord departed, and Abraham returned to his
place.
In this passage we see evidence of how God applies His Righteous Judgement. We still would concede it is of course, up to Him who He chooses to see as Righteous or Wicked for the purpose of Election so do no forget that; but we have more insight to how Merciful He is. He is so abounding in Grace, and it's a marvel that He saves anyone to begin with. More of His Word may help us to find more assurance in His character:
Matt 5-
5 “Blessed are the [a]gentle (meek, humble), for they shall inherit
the earth."
We must concede that Election is solely up to Him, that is sure; however, we can also see in principle that His character is reassuring as to how we theorize upon how He decides such a thing. There are no meeker, or gentle souls than those who have not a chance to perform actions; or that suffer from limited capacities. We also should remember, He is ultimately the cause of their existence, their condition, and their death.
I hope this provides a sort of reassurance, not that I can claim all lost children go to Heaven, but that we can find comfort in knowing the Mercy aspect of His character.
In other words, do your best not to worry over these situations - though it may be hard...
God will do exactly what is correct for every soul that exists, according to His own Will
Another alternate, but perhaps less helpful answer to the second question would be in short reply: God is not as you describe.