They simply return where they came from.That is to The Heavenly Father in heaven. All souls belong to God. and are sent from God.
Mark 10:13-15
King James Version (KJV)
13 And they brought young children to him, that he should touch them: and his disciples rebuked those that brought them.
14 But when Jesus saw it, he was much displeased, and said unto them, Suffer the little children to come unto me, and forbid them not: for of such is the kingdom of God.
15 Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.
Mark 9 :
36 And he took a child, and set him in the midst of them: and when he had taken him in his arms, he said unto them,
37 Whosoever shall receive one of such children in my name, receiveth me: and whosoever shall receive me, receiveth not me, but him that sent me.
42 And whosoever shall offend one of these little ones that believe in me, it is better for him that a millstone were hanged about his neck, and he were cast into the sea....
Ezekiel 18:4
King James Version (KJV)
4 Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: the soul that sinneth, it shall die.
( Read all of 1st Corinthians 15. ) . . .
2 Corinthians 5:8
King James Version (KJV)
8 We are confident, I say, and willing rather to be absent from the body, and to be present with the Lord.
Jeremiah 1:5
King James Version (KJV)
5 Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations.
We came from before God, we return there.
Also God is Judge and there is no other Judge beside Him.
Deuteronomy 32:
39 See now that I, even I, am he, and there is no god with me: I kill, and I make alive; I wound, and I heal: neither is there any that can deliver out of my hand.
40 For I lift up my hand to heaven, and say, I live for ever.
James 4:12
King James Version (KJV)
12 There is one lawgiver, who is able to save and to destroy: who art thou that judgest another?
I guess i glean from these few places that God takes care of the kids. So they are in good hands . There are so much more places you can read to get an good overall view of this , written in the Bible, I just don't know them all.